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EXPERIMENTAL 


PLANT  PHYSIOLOGY 


BY 

D.  T.   MACDOUGAL 

Univtrsity  of  Minnesota 


NEW  YORK 

HENRY  HOLT  AND  COMPANY 
1898 


M 


BIOL06Y 

LIBRARY 

6 


Copyright,  1895, 

BY 

HENRY  HOLT  &  Coc 


ROBERT  DRUMMOND.   ELECTROTYPER   AND   PRINTER,   NEW  YORK 


PREFACE. 


THE  appreciation  shown  toward  the  translation  of  Gels' 
Pflanzenphysiologische  Versuche  prepared  by  the  present  writer, 
together  with  the  comments  and  suggestions  from  laboratories 
in  which  it  has  been  used,  has  led  to  the  preparation  of  this 
manual,  which  it  is  hoped  will  conform  somewhat  more  nearly 
to  the  needs  of  American  students.  The  general  form  of 
Gels'  manual  has  been  retained,  and  many  cuts  from  the 
translation  and  a  few  paragraphs  of  the  text  have  been  re- 
peated here  without  indication  of  their  origin. 

Only  the  more  important  and  better  established  portions  of 
the  subject  are  treated,  and  these  in  the  manner  already  in  gen- 
eral use.  With  the  rapid  advance  of  investigation  it  is  next  to 
impossible  that  an  elementary  laboratory  manual  should  include 
the  latest  results,  especially  when  the  essential  points  of  many 
of  them  may  yet  be  in  controversy  and  need  the  critical  treat- 
ment which  is  certainly  not  within  the  province  of  a  work  of 
this  character.  In  the  hands  of  an  instructor  in  touch  with 
current  botanical  thought,  such  deficiencies  are  easily  supplied. 

In  the  interests  of  precision,  the  term  "  assimilation  "  is 
here  used  exclusively  to  denote  a  general  function  of  proto- 
plasm, while  the  term  "  photosynthesis,"  which  was  introduced 
into  the  translation  of  Oels'  manual  (Preface  and  page  30),  is 
adopted  to  signify  the  special  process  of  forming  carbohydrates 
from  carbon  dioxide  and  water  in  the  presence  of  chlorophyll 
and  sunlight. 

The  author  is  indebted  to  Mr.  R.  N.  Day  and  Miss  J.  E. 
Tilden  for  the  demonstration  and  drawing  for  Figure  32. 

D.  T.  MACD. 

MINNEAPOLIS,  MINN.,  April  15,  1895. 

iii 

258256 


CONTENTS. 


PAGE 

PREFACE iii 

INTRODUCTION i 

I.  ABSORPTION   OF   LIQUID   NUTRIMENT. 

Food  of  plants      ............     3 

Nutrient  elements 3 

Distilled  water  as  a  nutritive  fluid 6 

Influence  of  iron  .         .  .         .         .         .         .         .         .         .         .6 

Organs  of  absorption  ...........     7 

Zone  of  root-hairs         ...........     9 

Condition  of  nutrient  substances  in  the  soil II 

Nutrition  of  parasitic  plants        .         .         .         .         .         .         .         .         .10 

Nutrition  of  saprophytic  plants  .........   n 

Physical  aspects  of  plants 12 

Diffusion       ............  12 

Diffusion  through  epidermis 14 

Power  of  selection  of  food-material    ........  14 

Turgor 16 

II.  MOVEMENTS   OF  WATER   IN   THE   PLANT. 

Root  pressure       ............  ig> 

Transpiration        ............  20 

Evaporation  of  water  fiom  leaves       . 22 

Wilting  of  excised  shoots    ..........  23 

Conditions  of  transpiration          .........  24 

Cause  of  wilting  ............  26 

Guttation      .  ..........  27 

Attraction  of  soil  for  water          .........  27 

Uses  of  transpiration 28 

Ascent  of  sap        ............  28 

Path  of  sap  .............  33 

III.  ABSORPTION   OF  GASES. 

Gases  used  by  the  plant      ..........  35 

Diffusion  of  gases         ...........  36 

Absorption  of  gases     ...........  36 

v 


VI  CONTENTS. 

PAGE 

Photosynthesis 37 

Physical  properties  of  chlorophyll 39 

Division  of  the  spectrum     ..........  40 

Product  of  photosynthesis  ..........  41 

IV.  RESPIRATION    AND    OTHER   FORMS   OF  METABOLISM. 
Nature  of  metabolism .         .         .         .         .         .         .         .         .         .         .43 

Respiration     .............  43 

Absorption  of  oxygen  and  excretion  of  carbon  dioxide     .         .         .         .44 

Liberation  of  heat         ...........  45 

Respiration  essential  to  growth 46 

Fermentation .  47 

Changes  in  color 49 

V.  IRRITABILITY. 

Nature  of  irritability .50 

Perceptive  zone,  motor  zone 50 

Geotropism  .............   51 

Perceptive  and  motor  zones  of  roots .52 

Amount  of  influence  of  gravity 54 

Replacement  of  gravity .55 

Heliotropism,  thermotropism,  etc.      ........   57 

Periodic  movements 61 

Hydrotropism .62 

Contact  movements     ...........  62 

Circumnutation .66 

Hygroscopic  movements 66 

VI.  GROWTH. 

Nature  of  growth .68 

Grand  period  of  growth 70 

Influence  of  light  on  growth 72 

Influence  of  light  on  anatomy  of  leaves 73 

Influence  of  light  and  gravity  on  the  formation  of  organs         .         .         .74 
Influence  of  temperature  on  growth  ........  75 

Sources  of  heat 76 

Relation  of  temperature  to  distribution 76 

Freezing  of  plants        ...........   77 

Relation  of  moisture  to  freezing .78 

Lpss  of  heat .80 

Rresting  period 80 

Correlation  processes 81 

Mechanical  force  exerted  by  growing  organs     .         .         .         .         .         .81 

APPENDIX 84 

INDEX 87 


EXPERIMENTAL   PLANT   PHYSIOLOGY. 


INTRODUCTION. 

A  PLANT  is  a  living  organism  which  carries  on,  more  or 
less  constantly,  certain  life-processes.  The  more  important 
of  these  are  absorption  of  food-material*  photosynthesis,  respira- 
tion, transpiration,  secretion,  and  reproduction.  The  manner 
in  which  these  processes  are  performed  is  largely  determined 
by  the  influence  of  the  external  conditions  of  gravity,  light, 
heat,  moisture,  air,  climate,  etc. 

In  order  to  obtain  an  insight  into  plant  life  it  is  necessary 
to  consider  the  nature,  purpose,  and  mutual  interaction  of  the 
life-processes  involved  and  to  analyze  the  influence  exerted 
upon  them  by  environment. 

The  course  of  experiments  detailed  in  this  manual  deals 
only  with  the  more  salient  features  of  plant  physiology,  and 
is  illustrative  rather  than  quantitative.  In  some  instances, 
however,  the  simple  treatment  given  may  with  proper  applica- 
tion yield  exact  results.  The  physical  and  chemical  apparatus 
possessed  by  every  college  or  high  school  will  be  found  suf- 
ficient to  carry  out  the  work. 

Good  plant-material  is  absolutely  essential  to  the  profit- 
able performance  of  the  experiments  ;  and  unless  a  greenhouse 
is  at  hand,  the  course  should  be  pursued  at  a  time  when  plants 
may  be  grown  in  the  open  air. 


METHODS   OF  EXPERIMENTATION. 
The  following  books  will  be  found  useful  for  reference : 

DARWIN,  F.     Practical  Physiology  of  Plants.     1 894. 

GOODALE.     Physiological  Botany.     1884. 

KERNER  and  OLIVER.     Natural  History  of  Plants.     1894. 

SACHS.     Physiology  of  Plants.     1886. 

SPALDING.     Introduction  to  Botany.     1894. 

VINES.     Physiology  of  Plants.     1886. 

VINES.     Text-book  of  Botany.     1894-5. 


METHODS  OF  EXPERIMENTATION. 

THE  entire  course  'of  an  experiment  should  be  described  in 
detail  in  the  student's  note-book,  with  reference  to  the  follow- 
ing points : 

1.  Object  of  experiment. 

2.  Apparatus  and  plant-material  employed :  condition  and 
development  of  the  latter.     Full  drawings. 

3.  Date  of  experiment  and  successive  observations— day 
and  hour. 

4.  Temperature,  moisture,  and  sunshine. 

5.  Results  of  experiment. 


CHAPTER   I. 

ABSORPTION    OF    LIQUID   NUTRIMENT. 

1.  Food  of  Plants. — Green  plants  generally  derive  their  food 
from  simple  chemical  compounds  in  the  soil,  air,  and  water. 
Of  these   compounds  the  simple  mineral  salts  are  obtained 
from  the  soil.     Animal  manures  and  decaying  vegetable  mat- 
ter do  not   serve    directly  as   food-material.      Elements  and 
simple   compounds  liberated  by  the  decomposition  of  these 
substances  may  be  used  in  building  up  the  plant.     The  chief 
value  of  such  "organic"  matter  to  the  plant  lies  in  the  fact 
that  it  preserves  the  porous  condition  of  the  soil,  thus  allow- 
ing access  of  air  to  the  roots  and  retaining  water  containing 
nutrient  salts  in  solution.     The  decaying  "  organic  "  material 
in  the  soil  also  furnishes  the  proper  conditions  of  growth  for 
the  soil  Bacteria,  whose  activity  is  a  necessary  factor  in  the 
development  of  many  of  the  higher  plants.     An  admixture 
of  "  organic  "  material  with  the  mineral  elements  of  the  soil 
is  also  a  means  of  equalizing  the  temperature. 

Aquatic  plants  in  general  use  the  same  food-material  as 
land  plants.  The  water  in  which  they  grow  is  in  contact 
with  the  soil  and  contains  all  of  its  soluble  salts  in  solution. 

Remark. — It  is  to  be  noted  that  the  species  comprised  in  the  parasitic, 
saprophytic,  and  insectivorous  plants,  many  of  which  are  furnished  with 
chlorophyll,  are  able  to  use  directly  complex  substances  derived  from  plants 
or  animals,  and  do  not  depend  entirely,  or  at  all,  on  the  simple  compounds 
in  the  soil. 

2.  Nutrient  Elements. — In  order  to  determine  what  elements 
enter  into  the  food  of  plants,  and  the  office  of  each  substance> 


EXPERIMENTAL   PLANT  PHYSIOLOGY. 


plants  may  be  grown  in  solutions  of  different  composition.  It 
has  been  found  by  numerous  experiments  and  analyses  that 
only  potassium,  calcium,  magnesium,  sulphur,  phosphorus,  iron, 
and  sometimes  silica  and  chlorine  are  obtained  from  the  soil 
alone.  Of  the  remaining  substances  necessary  for  the  plant, 
hydrogen  is  obtained  from  both  water  and  the  soil  compounds, 
oxygen  from  both  the  soil  and  the  air,  nitrogen  usually  from 
the  soil,  but  in  some  instances  also  from  the  air,  and  carbon 
almost  entirely  from  the  air,  except  in  the  case  of  the  plants 
which  derive  it  from  complex  compounds.  (§§  I,  8,  and  9.) 

EXPERIMENT    i. 

WATER   CULTURES. 

Fill  a  large  bottle  with  distilled  water,  and  to  each  liter  of  water 

or 


add 

i.    gram  potassium  nitrate 
0.5     "      sodium  chloride 
0.5     "      calcium  sulphate 
0.5     "      magnesium  sulphate 
0.5     "      calcium  phosphate 
Warm  gently  for  an  hour  and  keep  in  a  dark  place. 

FIG.  i. 


i.     gram  calcium  nitrate 
0.25     "     potassium  chloride 
0.25  magnesium  sulphate 

0.25  acid  potassium  phos- 

phate 


Hansen's  germinator.  A,  glass  vessel  filled  with  water  and  covered  with 
tightly-stretched  bobbinet.  B,  bell-jar  fitted  with  moist  filter-paper. 
(Oels.) 


ABSORPTION   OF  LIQUID   NUTRIMENT. 


5 


Place  seeds  of  Wheat,  Corn,  Bean,  Pea,  or  Buckwheat  in  folds  of 
moist  cloth,  in  a  pan  of  moist  sawdust,  or  in  a  -Hansen  germinator 
(Fig.  i),  until  %the  radicles  are  a  centimeter  in  length. 

Fill  a  glass  jar  or  cylinder  of  a  capacity  of  i  to  2  liters  with  the 
solution  described  above.  The  bottle  should  be  well  shaken  before 
this  is  done.  Add  a  few  drops  of  iron  chloride  to  the  solution  in 
the  jar.  Cut  a  hole  i  cm.  in  diameter  through  the  center  of  a  large 
cork  and  fit  in  the  top  of  the  cylinder  or  jar  as  shown  in  Fig.  2. 

FIG.  2. 


Culture  cylinder  with  seedling  of  Corn  in  position.     K,  cork.     (Hansen.) 

Make  a  vertical  cut  from  the  outer  edge  of  the  cork  to  the  central 
opening.  Fasten  a  seedling  obtained  as  above  in  the  aperture  by 
means  of  cotton-wool  or  asbestos  fiber^  in  such  position  that  the 
root  only  is  immersed.  Set  in  a  sunny  place.  Renew  the  solution 
once  every  week. 

Allow  the  plant  to  grow  3  or  4  weeks  and  compare  with  others 
of  the  same  age  grown  in  the  soil. 


O  EXPERIMENTAL   PLANT  PHYSIOLOGY. 

Remark  1. — To  exclude  light  from  the  roots  and  prevent  the  growth  of 
Algae  in  the  culture  cylinder  it  should  be  fitted  with  a  jacket  of  pasteboard 
or  blackened  paper.  This  may  be  still  more  effectively  accomplished  if  the 
jar  is  sunk  its  entire  depth  in  the  soil  of  a  large  flower-pot  or  box.  If  the 
soil  is  watered  occasionally,  a  temperature  more  nearly  suitable  for  the 
roots  will  be  obtained. 

Remark  2. — Calcium  phosphate  is  only  slightly  soluble  in  water,  and  in 
consequence  it  forms  a  sediment  on  the  bottom  of  the  jar  which  decreases 
as  that  in  solution  is  used. 

Remark  3. — The  sodium  chloride  used  in  the  first  solution  is  not  of 
direct  use  to  the  plant,  but  serves  to  keep  the  solution  alkaline. 

Remark  4. — Analysis  and  agricultural  practice  show  that  plants  of  differ- 
ent species  and  genera  grown  in  the  same  soil  contain  the  elements  in  dif- 
ferent proportions  ;  consequently  a  solution  suitable  for  all  plants  cannot 
be  made.  Instead  of  the  salts  given  above,  others  which  contain  the  ele- 
ments in  soluble  form  may  be  used.  The  degree  of  concentration  must  be 
kept  within  the  prescribed  limit,  however. 

3.  Distilled  Water  as  a  Nutritive  Fluid. — A  plant  will  grow 
for  a  time  in  distilled  water,  but  when  the  food  stored  in  the 
seed  is  consumed  it  perishes. 

EXPERIMENT    2. 

DISTILLED   WATER   AS   A   NUTRITIVE   FLUID. 

Grow  two  seedlings  as  nearly  alike  as  possible,  one  in  distilled 
water  and  the  other  in  a  nutritive  solution  as  in  Experiment  i. 
Note  difference  in  10  and  14  days. 

4.  The  Influence  of  Iron. — The  plant  can  form  green  color- 
ing   matter   (chlorophyll)  only   when  supplied  with   iron.     If 
this  is  withheld,  the  plant  dies  after  it  has  used  the  iron  stored 
in  the  seed.     The  presence  of  chlorophyll  is  necessary  for  the 
formation  of  food  from  the  carbon  dioxide  of  the  air.     (§  29.) 

EXPERIMENT    3. 

IRON-FREE   NUTRITIVE    SOLUTION. 

Grow  two  plants  in  an  iron-free  nutritive  solution.  The  first 
leaves  are  green  and  the  later  ones  pale  yellow  (chlorotic), 

EXPERIMENT    4. 

ADDITION    OF    IRON   TO    A    CHLOROTIC    PLANT. 

Pour  a  few  drops  of  iron  solution  into  the  culture  jar  of  a  chlo- 
rotic  plant  obtained  by  Experiment  3.  The  leaves  soon  become 
green.  With  a  small  brush  moisten  portions  of  a  leaf  of  another 


ABSORPTION   OF  LIQUID   NUTRIMENT.  7 

chlorotic  plant  with  iron  solution.  The  portions  treated  will  become 
green  in  a  short  time,  and  this  color  will  gradually  extend  over  the 
plant. 

5.  Organs  of  Absorption. — In  the  lower  plants  ot  simple  or- 
ganization the  absorption  of  nutriment  is  carried  on  by  a  greater 
part  or  all  of  the  surface  of  the  organism.  In  the  higher 
plants  the  roots  are  the  special  organs  of  absorption,  and 
nearly  all  of  the  liquid  taken  in  by  the  plant  is-  obtained 
through  them.  The  marked  branching  shown  in  the  root  sys- 

FIG.  3. 


Cross-section  of  a  root  showing  structure  and  arrangement  of  root-hairs. 
The  latter  are  swollen  in  places,  applying  a  broader  surface  to  the  soil- 
particles  in  contact  with  them.  (Frank.) 

tern  of  the  higher  plants  not  only  increases  the  efficiency  of 
the  roots  as  organs  for  the  fixing  of  the  plant  in  the  soil,  but 
also  magnifies  the  absorbing  surface.  Absorption  is  carried 
on  through  the  outer  walls  of  the  peripheral  cells  which  con- 
stitute the  epidermis.  In  land  plants  the  outer  walls  of  these 
epidermal  cells  are  developed  into  long  tubelike  extensions, 


8 


EXPERIMENTAL   PLANT  PHYSIOLOGY. 


the  root-hairs  (Fig.  3).  The  amount  of  surface  extension  ob- 
tained by  root-hairs  is  very  great,  since  these  structures  are 
.008  mm.  to  .14  mm.  in  diameter,  and  often  attain  a  length  of 
3  mm.,  while  from  10  to  400  may  be  formed  on  a  square 
millimeter  of  surface. 

The  root-hairs  are  also  an  adaptation  for  obtaining  water 
FIG.  4.  under   the    conditions   in  which   it   is 

found  in  the  soil,  where  it  occurs  in  the 
form  of  a  minute  layer  on  the  surface 
of  the  soil-particles.  The  root-hairs 
are  capable  of  bending  around  and 
penetrating  between  the  particles  in  a 
manner  which  places  their  walls  in  con- 
tact with  a  large  amount  of  this  layer 
of  water.  In  aquatic  plants  root-hairs 
are  not  needed,  and  are  rarely  formed, 
since  the  entire  body  of  the  root  is  in 
contact  with  the  water.  It  will  be  seen 
that  the  land  plants  grown  in  water  in 
the  culture  experiments  developed  very 
few  root-hairs.  On  the  other  hand, 
plants  grown  in  dry  soil  exhibit  a  very 
marked  development  of  these  struc- 
tures. In  this  instance  tthe  amount  of 

Seedlings  of  White    Mus-  water  around  each  soil-particle  is  very 
tard.     (Sachs.)     A,   with 
soil  clinging  to  the  roots  ;  small,    and   the   plant   must    reach    a 


much  larger  number  of  them  in  order 
to  obtain  the  needed  supply. 

EXPERIMENT    5. 

ADHESION   OF    ROOT-HAIRS    TO    SOIL-PARTICLES. 

Grow  seedlings  of  Mustard,  Pea,  or  Corn  in  sandy  soil.  When 
one  week  old  take  up  and  note  amount  of  soil  clinging  to  roots 
(Fig.  4).  Free  from  the  mass  of  the  soil  by  washing.  Examine 


ABSORPTION   OF  LIQUID   NUTRIMENT.      .  9 

with  a  magnification  of  10  to  25  diameters  and  observe  the  remain- 
ing soil-particles  attached  to  the  irregular  root-hairs. 

EXPERIMENT    6. 

STRUCTURE   OF    ROOT-HAIRS. 

Cut  a  thin  cross-section  of  the  root  of  a  seedling  grown  in  the 
germinator  and  examine  with  a  magnification  of  50  to  100  diameters. 
Note  the  tubelike  structure  of  the  hairs,  the  thin  irregular  layer  of 
protoplasm  on  the  inner  side  of  the  wall  and  the  large  transparent 
central  portion  filled  with  sap.  Examine  the  base  of  the  hair  and 
note  its  relation  to  the  neighboring  cells  of  the  root  (Fig.  3). 

6.  Zone  of  Root-hairs. — As  the  root  extends  in  length  by 
the  growth  of  a  portion  near  the  tip,  new  root-hairs  constantly 
arise  in  this  region,  while  the  older  ones  in  the  region  farther 
away  are  constantly  dying.  The  zone  of  root-hairs  ordinarily 
begins  I  to  3  cm.  back  of  the  tip  and  extends  backward  along 
the  root  for  a  distance  of  about  5  to  FIG.  5. 

10  cm.  In  this  manner  new  root-hairs 
are  continually  brought  into  contact 
with  fresh  particles  of  soil. 

EXPERIMENT   7. 

MOVEMENT   OF    ZONE    OF    ROOT-HAIRS. 


Place  a  germinated  seed  of  Pea  or 
Squash  in  a  small  funnel  or  thistle-tube 
in  such  manner  that  the  root  extends 
downward  in  the  narrow  outlet.  Cover 
the  seed  with  moist  cotton  and  place  a 
layer  of  moist  filter-paper  and  a  glass 

plate  over  the  top  of  the  funnel  to  prevent  A 

Apparatus  to  demonstrate 

the  seedling  from   becoming   dried.      Set     progression  of  zone    of 

the  funnel  upright  in  a  bottle  containing     root-hairs.   (After  Dels.) 

A,     potassium      hydrate 

a  small  amount  of  a  solution  of  potassium      solution  ;  L,  moist  filter- 
hydrate.       This   solution   will    absorb    the     PaPer- 
carbon  dioxide  gas  given  off  by  the  seedling.     By  means  of  India 
ink  mark  on  the  glass  tube  the  boundaries   of    the  zone  of  root- 
hairs  every  day  during  a  week.     (Fig.  5.) 


10 


EXPERIMENTAL   PLANT  PHYSIOLOGY. 


FIG.  6. 


7.  Condition  of  Nutrient  Substances. — Only  liquid  nutriment 
is  taken  up  by  the  roots.     The  mineral  substances  of  the  soil 
are  slowly  dissolved  by  percolating  water  which  contains  small 
amounts  of  oxygen  and  carbon  dioxide,  as  well  as  traces  of 
nitric  and  sulphuric  acids  derived  from  the  air.    In  some  cases 
the  walls  of  the  root-hairs  are  saturated  with  an  acid   sap, 
which  aids  in  the  solution  of  the  mineral  salts. 

EXPERIMENT    8. 

ACIDITY  OF  ROOTS. 

Place  the  roots  of  a  seedling  of  Pea,  Bean,  or  Corn  grown  in  a 

germinator,  on  a  sheet  of  blue  litmus 
paper.  The  portion  of  the  paper 
touched  becomes  red,  indicating  the 
presence  of  an  acid. 

EXPERIMENT  9. 

CORROiWE  ACTION   OF   ROOTS. 

Fill  a  5-inch  pot  half  full  of 
moist  loam.  On  this  lay  a  piece  of 
marble  whose  upper  surface  is  highly 
polished.  Fill  the  pot  with  moist 
sand  and  imbed  a  germinated  pea 
or  bean  near  the  surface.  After  the 

soil  has  been  thoroughly  penetrated 
Marble  platea>rroded  by  roots.     by  the  rootg    (IQ    tQ    ^   dayg)   take 

out  the  marble  plate,  dry,  and  by  re- 
flected light  note  the  rough  lines  etched  on  its  upper  surface  by  the 
acid  of  the  roots. 

8.  Nutrition  of  Parasitic    Plants. — Many   plants    of   which 
Mistletoe  and  Cuscuta  are  examples  do  not  develop   a  root 
system  for  the  absorption   of  nutriment    from   the  soil,   but 
attach  themselves  to  the  bodies  of  other  plants,  from  which 
they  derive  sap  containing  the  necessary  substances  already 
prepared.     Such  plants  are  termed  parasites.     In  many  cases 
parasitic  plants  are  entirely  devoid  of  chlorophyll  and  depend 
altogether  on  the  host-plant  for  their  food.     Cuscuta  (Dodder), 


ABSORPTION   OF  LIQUID    NUTRIMENT.  II 

Epiphegus  (Beechdrops),  and  the  microscopic  Rusts  and  Smuts 
are  examples  of  plants  of  this  latter  type.  Mistletoe  and  many 
other  parasitic  plants  are  furnished  with  chlorophyll  and  are 
able  to  obtain  a  portion  of  their  food-supply  from  the  simple 
compounds. 

EXPERIMENT    10. 

NUTRITION   OF   CUSCUTA. 

Examine  plants  of  any  ordinary  species  of  Impatiens  growing 
in  wet  or  swampy  ground,  in  September.  On  some  of  these  plants 
may  be  found  the  yellowish  cordlike  twining  stems  of  Cuscuta, 
bearing  knotty  masses  of  pale  yellow  or  cream-colored  flowers. 
With  the  plants  still  in  position  note  that  the  Cuscuta  has  no  soil- 
roots,  but  that  it  sends  short  haustoria  or  suckers  into  the  stem  of 
the  Impatiens.  With  a  sharp  knife  cut  across  the  stem  of  the  host- 
plant  and  determine  the  depth  to  which  the  haustoria  have  pene- 
trated. The  haustoria  obtain  sap  from  the  host-plant  by  osmose,  in 
a  manner  similar  to  the  action  of  the  root-hairs  of  land  plants. 

9.  Nutrition  of  Saprophytic  Plants.  —  Many  plants  derive 
all  or  a  large  proportion  of  their  food-material  from  the 
products  of  the  metabolism  (see  Chapter  IV)  of  other  organ 
isms.  Such  plants  are  termed  saprophytes.  Examples  of  this 
type  are  afforded  by  Corallorhiza  (Coral-root),  Monotropa 
(Indian-pipe),  Toadstools,  Mushrooms,  and  many  Bacteria. 
EXPERIMENT  u. 

NUTRITION   OF    TOADSTOOLS. 

Note  the  growth  of  Toadstools  and  other  Fungi  on  pieces 
of  decaying  wood  in  a  damp  forest.  Tear  apart  the  mass  on  which 
the  plants  are  growing,  and  trace  the  long  irregular  absorbent  organs 
ramifying  in  all  directions  through  the  mass. 

EXPERIMENT  12. 

NUTRITION   OF   MOULDS. 

Place  a  fragment  of  saturated  bread  under  a  moist  bell-jar  for 
two  days.  A  number  of  slender  hyphce  of  a  Mould  may  be  seen 
springing  from  the  bread.  Tear  apart  a  small  bit  of  the  bread 
and  examine  with  a  magnification  of  50  diameters.  The  absorbent 
mycelia  can  be  seen  branching  in  all  directions. 


12  EXPERIMENTAL   PLANT  PHYSIOLOGY. 

EXPERIMENT  13. 

NUTRITION   OF   BACTERIA   AND    RELATED    FORMS. 

Make  a  solution  of  sugar  in  a  cylinder  and  set  in  a  warm  room 
for  three  days.  A  film  or  scum  will  be  formed  on  the  surface  of 
the  liquid.  Under  a  magnification  of  600  diameters  the  scum  will 
be  found  to  consist  of  an  immense  number  of  globular,  cylindrical, 
or  spiral  cells  of  Bacteria  and  other  forms  which  obtain  their  food 
from  sugar  and  complex  substances  formed  by  other  plants. 
These  organisms  absorb  food-material  through  their  entire  surface. 
The  spores  of  such  plants  are  found  floating  in  the  air  and  develop 
whenever  they  come  in  contact  with  food  under  proper  conditions 
of  temperature. 

10.  Physical   Aspects  of  a  Plant. — From  a  purely  physical 
point    of   view   the    plant    may  be  regarded    as  a  cylindrical 
chamber  whose  walls  are  composed  of  membrane,  and  whose 
contents  consists  of   a  large  number  of   stable  and  unstable 
compounds  dissolved  in  water.     At  both  ends  of  the  cylinder 
the  surface  is  magnified,  at  the  lower  end   in  the    roots   and 
root-hairs,  and  at  the  upper  end  in  the  leaves,  to  facilitate  the 
diffusion  of  gases  and  liquids.     The  body  of  the  cylinder,  the 
stem,  acts   as   a   tubelike  conductor   between   these  surfaces. 
The  outer  layer  of  the  stem  is  not  easily  permeable  by  fluids. 

11.  Diffusion. — By   diffusion    is    understood    any   exchange 
which   may  take   place   between   two  fluids  in  contact  either 
directly  or  through    a    membrane.     This   latter   exchange    is 
termed  osmose.     Diffusion    takes   place   regardless  of   gravity 
until  the  fluids  are  alike.     Not  all  fluids  are  capable  of  osmose, 
but    only   those   which    are    imbibed    by   a    membrane.     The 
rapidity  of   diffusion    varies  with    the  mobility  of   the  fluids. 
Stable    compounds   diffuse   with    more    difficulty  than  water. 
Concentrated  solutions  of  these  compounds  increase  in  volume, 
since  they  gain  more  water  than  they  lose.     They  occur  in  the 
root-hairs,  and   in    consequence  a  large  quantity  of   water  is 
taken  up  and  forced  into  the  root  and  upward  in  the  stem. 


ABSORPTION  OF  LIQUID   NUTRIMENT. 


EXPERIMENT  14. 

OSMOTIC   ACTION   OF   A   SUGAR    SOLUTION. 

Cover  the  large  end  of  a  thistle-tube  or  small  glass  funnel  with 
tightly-stretched  membrane,  such  as  parchment  or  bladder,  which 
has  been  soaked  for  15  minutes  in  water.  Fill  the  large  part  of  the 
tube  or  funnel  with  a  solution  of  sugar  i  part  and  water  3  parts,  and 
fasten  upright  by  means  of  a  large  perforated  stopper  in  a  cylinder 
containing  water,  in  such  position  that  the  two  fluids  are  on  a  level. 
Note  the  height  of  the  solution  in  the  tube  in  12  and  24  hours.  A 
large  amount  of  water  has  been  drawn  through  the  membrane  into 
the  sugar  solution,  while  only  a  small  portion  of  the  latter  has  passed 

out  into  the  cylinder,  as  can  be  ascertained  by 

tasting.     (Fig.  7.) 

EXPERIMENT    15. 

OSMOTIC  ACTION   OF   A    SOLUTION  OF  COPPER  SULPHATE. 

Cover  one  end  of 
an  ordinary  lamp-chim- 
ney tightly  with  hog's 
bladder  or  parchment, 
fill  with  a  solution  of 
copper  sulphate,  and 
suspend  in  a  vessel  of 

FIG.  9. 


FIG.  7. 


FIG.  8. 


Osmometer.  (Miil- 
ler.)  b,  bulb  of 
thistle  -tube  ;  Z>, 
level  of  liquid  in 
cylinder;  r,  levelof 
liquid  in  tube  after 

donW  h°UrS'  °Pera"    distilled  water. 


Carrot   hollowed 
out    and    filled 
with    sugar. 
(Mttllcr.) 


Osmometer.  (Oels.) 
K,  cork  to  hold 
lamp-chimney  in 
place  ;  c,  open  end 
of  lamp-chimney. 

After  a  time  the  bluish  color 
of  the  water  in  the  vessel,  and  the  copper-red 
coating  formed  on  an  iron  nail  placed  in  the  vessel,  denote  that  the 
copper-sulphate  solution  has  passed  through  the  membrane  into  the 
distilled  water.  (Fig.  8.) 


H  EXPERIMENTAL   PLANT  PHYSIOLOGY. 

EXPERIMENT    16. 

OSMOSE    IN    PLANT-TISSUES. 

Hollow  out  the  central  part  of  a  large  Carrot,  making  the  walls 
of  the  cavity  formed  about  .5  cm.  in  thickness.  Fill  the  cavity  with 
dry  sugar.  Twenty-four  hours  later  the  sugar  will  be  dissolved  in 
the  sap  which  is  drawn  into  the  cavity,  while  the  Carrot  is  dry  and 
shrunken. 

12.  Diffusion  through  Epidermis  of  Aerial  Organs. — Roots  and 
root-hairs  are  pre-eminently  organs  of  absorption,  yet  in  some 
instances  leaves  and  stems  exercise  this  function.     The  leaf- 
like  organs  of  Mosses  and  Liverworts  are  capable  of  absorbing 
water. 

EXPERIMENT  17. 

WATERPROOFING    OF   LEAVES. 

Cut  off  a  leaf  of  the  Cabbage,  Oak,  Beech,  or  Iris  and  immerse 
in  water.  The  surface  takes  on  a  silvery  appearance,  due  to  the 
thin  layer  of  air  adhering  to  it.  An  examination  will  show  a 
heavy  layer  of  cuticle  or  waxy  substance  on  the  outer  side  of  the 
epidermis. 

EXPERIMENT  18. 

ABSORPTION   OF   WATER   BY    LEAVES. 

Cut  off  a  young  branch  of  Coleus,  Geranium,  Tomato,  Impatiens 
or  other  convenient  plant  and  seal  the  end  with  wax  or  gum.  Lay 
aside  until  slightly  wilted.  Immerse  entirely  in  a  vessel  of  water. 
Examine  in  two  hours.  If  the  leaves  are  capable  of  absorbing 
water,  they  will  be  restored  to  their  original  condition.  It  will  be 
found  that  few  plants  can  take  water  by  means  of  the  leaves.  A 
moist  atmosphere  prevents  loss  of  water,  but  does  not  form  a  source 
of  supply  for  the  plant. 

13.  Power  of  Selection  of  Food-material. — The  root-hairs  are 
immersed  in  a  solution  of  mineral  salts  in  the  soil  in  a  manner 
similar  to  the  thistle-tube  in  Experiment  14.     By  the  laws  of 
diffusion  all  of  these  substances  should  be  absorbed  by  the 
root-hairs  until  an  osmotic  equilibrium  is  established,  and  gen- 


ABSORPTION   OF  LIQUID   NUTRIMENT. 


erally  such  is  the  case.  The  amount  of  any  one  substance 
necessary  to  establish  equilibrium  is  very  small,  and  as  soon  as 
this  amount  is  acquired,  absorption  of  that  substance  ceases. 
When  the  plant  withdraws  any  of  the  substances  from  the  cell- 
sap  solution  to  build  up  tissue,  another  quantity  of  that  sub- 
stance is  absorbed  from  the  soil.  Thus  different  quantities  of 
the  various  substances  are  absorbed.  It  is  to  be  noted  that 
all  substances  in  the  soil  are  not  invariably  extracted  even  in 
the  minutest  quantity  by  any  one  plant.  The  "  rotation  of 
crops  "  has  its  value  for  the  farmer  because  different  plants  do 
not  require  the  same  soil-salts. 

EXPERIMENT  19. 


INCREASED    DIFFUSION. 


Close  the   lower  end  of   two   lamp-chimneys  with  bladder   or 

FIG.  10. 


Apparatus  to  show  selective  diffusion.     (After  Oels.)     K K,  corks,  loosely 
fitted  ;    C  C,  copper-sulphate  solution. 

parchment  and  fill  with  distilled  water.  In  the  upper  end  of  one 
cylinder  place  a  stopper  into  which  several  iron  nails  have  been 
driven.  Make  a  saturated  solution  of  copper  sulphate  and  place 
exactly  the  same  amounts  in  both  cylinders.  Now  fasten  the  two 
chimneys  in  the  cylinder  as  shown  in  Fig.  10.  In  48  hours  take  out 


16 


EXPERIMENTAL   PLANT  PHYSIOLOGY. 


the  chimneys  and  note  that  a  bright  deposit  of  copper  has  formed 
on  the  nails.  Pour  some  granulated  zinc  into  each  cylinder.  Iii 
24  hours  take  out  the  undissolved  zinc,  and  filter  the  solution  in 
both  jars,  to  obtain  the  copper  precipitate.  Allow  the  precipi- 
tate to  remain  on  the  filter-paper  and  dry.  Weigh.  It  will  be 
found  that  a  much  smaller  quantity  is  obtained  from  the  solu- 
tion in  the  apparatus  containing  the  nails.  The  action  of  the 
iron  nails  in  withdrawing  the  copper  from  the  solution  inside  the 
lamp-chimney,  thus  causing  an  additional  amount  to  be  taken  up 
from  the  outside,  will  show  the  manner  in  which  a  plant  exercises 
a  "  selective  power  "  of  absorption. 

14.  Turgor. — When  a  living  cell,  composed  of  protoplasm 
enclosing  the  cell-sap  and  surrounded  by 
the  wall,  is  placed  in  contact  with  water  it 
absorbs  the  water  in  such  quantity  that  the 
wall  is  stretched,  while  on  the  other  hand 
the  wall  tends  to  contract  by  its  own  elas- 
ticity. Thus  a  cell-tension  is  set  up  which 
is  denoted  turgor  (Fig.  n).  The  cells 
composing  many  of  the  tissues  do  not 
absorb  water,  while  others  take  up  large 
quantities  and  expand  in  consequence. 
If  now  a  tissue  which  absorbs  much  water 

,     ..     ,TT      is  attached  to  another  which  remains  pas- 
Diagram  of  cell.     (Har- 

tig.)    a,  c,  wall;    6,  sive>  a  strain,  or  tissue-tension,  will  be  set 
protoplasm  ;  d,  nucle- 
us ;  e,  cell-sap.  up.     These    tissue-tensions    give    rigidity 

to  herbaceous  plants.  The  wilting  of  plants  is  accompanied 
by  loss  of  turgor,  and  consequent  decrease  of  the  tissue- 
tensions. 

EXPERIMENT  20. 

TURGOR   IN   AN   ARTIFICIAL   CELL. 

Cover  one  end  of  an  open  glass  cylinder  10  cm.  in  length  (a 
large  tube  will  suffice)  with  membrane,  fill  with  a  sugar  solution, 
close  the  other  end  in  the  same  manner  and  place  in  a  vessel 


ABSORPTION   OF  LIQUID   NUTRIMENT. 


containing  water.  The  contents  of  the  cylinder  increase  in  volume 
by  absorption  of  water,  and  the  membranes  take  a  convex  form 
in  consequence  of  the  increased  pressure  inside  the  cylinder. 
Place  the  cylinder  in  a  vertical  position  and  pierce  the  upper  mem- 
brane with  a  needle.  The  liquid  spurts  upward  from  the  pressure. 

(Fig.  12.) 

FIG.  12. 


Artificial  cell  to  illustrate  force  of  turgor.     (After  Oels.) 
EXPERIMENT  21. 

IMBIBITION   (OSMOSE)   OF   WATER   BY   SEEDS. 

Ascertain  the  exact  weight  of  20  dried  Peas,  and  place  in  a 
dish  containing  distilled  water.  In  24  hours  the  Peas  will  have 
greatly  increased  in  size  by  the  diffusion  of  water  through  the 
seed-coat.  The  substances  stored  in  the  seed  have  a  strong  attrac- 
tion for  water.  Dry  the  seeds  by  rubbing  with  a  cloth,  and  weigh. 
In  some  cases  they  will  have  taken  up  their  own  weight  of  water. 
The  force  of  the  osmose  may  be  shown  if  a  bottle  of  25  cc.  capacity 
is  filled  with  the  seeds  and  immersed  in  a  vessel  of  water  for  24 
hours. 

EXPERIMENT  22. 

LONGITUDINAL   TISSUE-TENSIONS. 

Cut  a  slice   a   centimeter   in  thickness   and  10  cm.   in  length 

FIG.  13. 


m 


m 


Longitudinal  tissue-tensions.  (Hansen.)  a,  outward  curvature  of  a  sec- 
tion due  to  excess  of  turgor  of  pith  ;  £,  length  of  separated  tissues  ; 
m  m,  pith  (parenchyma). 


18 


EXPERIMENTAL   PLANT  PHYSIOLOGY. 


FIG.  14. 


from  the  centre  of  a  young  branch  of  Elder  (Sambucus)  or  stem 
of  Rhubarb.  Divide  the  slice  in  halves  and  note  the  outward 
curvature  of  the  two  parts.  Describe  the 
tensions  which  existed.  Prepare  another 
slice  and  separate  the  parenchyma  (pith) 
from  the  wood  and  epidermis.  The  pa- 
renchyma expands  and  the  other  portions 
contract.  (Fig.  13.) 

EXPERIMENT  23. 


TRANSVERSE   TISSUE-TENSIONS. 


Transverse    tissue-ten- 
sions.    (Detmer.) 


Cut  a  ring  of  bark  from  a  young  twig  of 
Willow  or  Poplar,  and  after  a  few  minutes 
replace  in  its  original  position.  It  now  does  not  extend  entirely 
around  the  twig.  When  in  that  position  it  must  have  been  in  a 
stretched  condition.  (Fig.  14.) 


CHAPTER  II. 

MOVEMENTS    OF   WATER   IN   THE   PLANT. 

15.  Root-pressure. — The  roots,  by  reason  of  the  osmotic 
activity  of  the  substances  which  they  contain,  are  constantly 
absorbing  water.  The  amount  taken  up  during  the  winter 
season  when  the  soil  is  either  frozen  or  at  a  very  low  tempera- 
ture is  very  small.  At  the  beginning  of  spring  the  storage 
products  which  were  accumulated  in  the  roots  during  the  latter 
part  of  the  previous  season  are  changed  into  substances,  such  as 
sugar,  dextrine,  asparagin,  etc.,  which  are  soluble  and  possess 
great  osmotic  activity.  At  this  season  the  leaves  are  not  yet 
formed,  and  only  a  limited  amount  of  water  is  carried  up  and 
transpired  by  the  plant ;  consequently  the  water  taken  in  by 
the  roots  is  slowly  forced  upward  in  a  stream,  almost  filling 
the  wood-cells  in  the  lower  part  of  the  plant.  The  action  of 
the  roots  is  well  illustrated  by  the  osmometer  described  in 
Experiment  14.  The  pressure  with  which  the  water  is  forced 
upward  by  a  Nettle  will  sustain  a  column  of  water  3  or  4  meters 
high.  In  the  Grape  the  root-pressure  is  sufficient  to  sustain  a 
column  of  water  10  meters  in  height.  A  yearly  periodicity 
of  root-pressure  is  noticed  in  trees  and  other  perennial  plants. 
In  addition  it  can  be  demonstrated  that  daily  variations  due 
to  temperature  of  soil  and  air  and  the  humidity  of  the  air  occur. 
In  the  Grape  the  pressure  is  greatest  in  the  forenoon,  and 
decreases  from  12  to  6  P.M.  The  root-pressure  of  the  Sunflower 
reaches  its  maximum  and  begins  to  decrease  at  10  A.M. 

19 


20 


EXPERIMENTAL   PLANT  PHYSIOLOGY. 


EXPERIMENT  24. 

MEASUREMENT     OF     ROOT-PRESSURE. 

Cut  off  the  stem  of  an  actively-growing  plant  of  Dahlia,  Geranium, 
Corn,  Sunflower,  or  Grape  a  short  distance  above  the  ground,  and 
fasten  tightly  to  the  stump  in  a  perpen-  pIG.  I5> 

dicular  position  a  long  glass  tube  by 
means  of  a  short  section  of  rubber  tubing. 
Observe  the  varying  height  of  the  sap 
in  the  tube  from  day  to  day,  noting  the 
temperature  and  moisture  of  the  air  at  the 
same  time.  (Fig.  15.) 

16.  Transpiration. — The  water  taken 
up  by  the  roots  finds  its  way  upward 
through  the  stem  toward  the  leaves, 
where  a  constant  diffusion  into  the  air 
takes  place.  The  diffusion  of  water 
from  the  leaf  or  other  organs  of  a 
plant  into  the  air  is  designated  trans- 
piration. \  Transpiration  takes  place 
under  the  same  physical  laws  as  the 
evaporation  of  water  from  a  moist 
membrane.  Barometric  pressure,  light, 
temperature,  humidity,  and  move- 
ments of  the  air  are  the  most  impor- 
tant conditions  affecting  the  process.  Apparatus  for  demonstra- 
tion of  root-pressure. 

The  amount  of  water  actually  given  off  (Detmer.) 
varies  also  with  certain  metabolic  processes  (see  Chapter  IV.). 
A  plant  may  be  compared  to  a  tube  filled  with  water,  with 
an  expanded  upper  end  closed  by  a  membrane,  while  the 
lower  end  is  immersed  in  water.  By  evaporation  an  upward- 
flowing  stream  is  set  in  motion. 

EXPERIMENT  25. 

LIFTING-POWER    OF    THE    EVAPORATION   OF    WATER    FROM    A   MEMBRANE. 

Fill  a  thistle-tube  by  placing  it  in  a  vessel  of  water,  and  while  in 
that  position  cover  the  large  end  by  a  tightly-stretched  membrane. 


MOVEMENTS   OF    WATER   IN    THE  PLANT. 


21 


Close  the  small  end  of  the  tube  with  the  finger,  lift  from  the  water, 
and  place  in  an. upright  position  with  the  small  end  immersed  in  a 
dish  of  mercury.  (Fig.  16.)  Examine  daily  for  two  weeks  or  more. 
As  the  water  evaporates  the  mercury  slowly  rises.  It  may  be  drawn 
to  a  height  of  34  cm.  if  a  good  quality  of  ox-bladder  is  used. 

FIG.  17. 


FIG.  16. 


Apparatus  to  demonstrate  lifting  power  of  evaporation.     (After  Oels.) 

This  experiment  may  also  be  carried  on  in  the  following  manner 
to  determine  the  amount  of  water  evaporated  :  Fit  a  thistle-tube 
with  a  membrane  as  above,  and  while  still  under  water  attach  to  it 
by  means  of  a  short  section  of  rubber  tubing  a  glass  tube  bent  twice 
at  right  angles  (Fig.  17).  Completely  fill  the  apparatus  with  water 
and  fasten  in  an  upright  position.  Place  a  drop  of  oil  on  the  surface 
of  the  water  in  the  open  tube  to  prevent  evaporation  at  this  point. 
The  amount  of  evaporation  from  the  membrane  will  be  shown 
directly  by  the  fall  of  the  level  in  the  open  tube. 


22 


EXPERIMENTAL   PLANT  PHYSIOLOGY. 


17.  Water  Evaporates  from  Leaves  as  from  a  Membrane. — The 
shoots  and  leaves  of  plants  give  off  water  in  a  manner  siinilai 
to  the  action  of  the  apparatus  in  the  above  experiments. 
EXPERIMENT  26. 

LIFTING-POWER    OF    TRANSPIRATION. 

By  means  of  a  closely-fitting  rubber  stopper  fasten  a  leafy  shoot 
of  a  woody  plant  (Raspberry,  Rosebush,  etc.)  in  one  end  of  a  U  tube 
filled  with  water  and  FIG.  19. 

mercury.  The  mercury 
in  the  open  arm  of  the 
tube  soon  begins  to  sink, 
indicating  a  loss  of  water 
from  the  leaves.  (Fig. 
1 8.)  This  fact  may  alsc 
be  demonstrated  by  fit- 
ting the  shoot  to  the 
upper  end  of  a  straight 
tube  whose  lower  end  is 

FIG.  18. 


Lifting  power  of  transpira- 
tion. (After  Oels.)  a, 
water;  b,  mercury. 


Lifting  power  of  transpiration. 
(Detmer.) 


MOVEMENTS   OF   WATER  IN   THE  PLANT.  2$ 

immersed  in  mercury.      (Fig.  19.)     The  lifting  power  of  transpira- 
tion can  be  estimated  from  the  height  of  the  mercury  column. 

EXPERIMENT  27. 

ESTIMATION  OF   THE   TRANSPIRATION    FROM    A    SINGLE    LEAF. 

Fasten  a  leaf  with  around  smooth  petiole  in  one  end  of  a  U  tube 
by  means  of  a  rubber  stopper.  Previously  fill  the  U  tube  with 
water  and  fit  in  the  other  end  a  long  capillary  tube  bent  at  right 
angles.  Place  the  apparatus  in  such  position  that  the  leaf  will  be 


FIG.  20. 
b 


Apparatus  for  estimation  of  trans- 
piration. (Mangin.)  The  water 
recedes  from  a  toward  b. 


held  upright,  and  the  long  arm  of  the  small  tube  horizontal  (Fig. 
20).  A  small  amount  of  transpiration  from  the  leaf  will  cause  the 
water  in  the  small  tube  to  recede  horizontally.  The  amount  and 
rate  of  transpiration  may  be  easily  computed. 

18.  Wilting  of  Excised  Shoots  in  Water.  —  Herbaceous 
shoots  when  cut  off  and  set  in  water  generally  wilt  quickly, 
but  if  the  shoot  is  cut  under  water  it  remains  fresh  a  much 
longer  time.  Evidently  the  cause  of  the  wilting  when  the 
stems  are  cut  without  this  precaution  is  the  penetration  of  the 
shoot  by  air.  Perhaps  in  rapidly-growing  plants  escaping  slime 
may  seal  up  the  ends  of  the  vessels  which  conduct  water. 

EXPERIMENT  28. 

WILTING   OF   SHOOTS    EXCISED    IN   AND    OUT   OF   WATER. 

Bend  a  long  shoot  of  a  slightly  woody  plant  (Symphytum,  Rose- 
bush, etc.),  so  that  a  portion  of  the  stem  is  under  the  surface  of  the 


24  EXPERIMENTAL   PLANT  PHYSIOLOGY. 

water  in  a  dish.  Cut  off  the  stem  under  water,  and  it  will  remain 
fresh  several  days  if  the  cut  end  is  kept  immersed.  At  the  same 
time  cut  off  another  shoot  in  the  air,  and  after  10  minutes  place  the 

FIG.  21. 


Excision  of  a  shoot  under  water.     (After  Oels.) 

cut  end  in   the   vessel  of  water  with   the  other  shoot.     Compare 
results  daily.     (Fig.  21.) 

19.  Conditions  of  Transpiration. — Plants  transpire  water  con- 
stantly over  their  entire  aerial  surface,  yet  the  stems  and  the 

FIG.  22. 

c 


Stoma  from  under  side  of  leaf  of  Iris  florentina.    c,  cuticle.    (Strasburger.) 

greater  part  of  the  leaves  are  covered  with  an  almost  impervious 
layer  of  cuticle.  Beside  this,  the  devices  exhibited  by  plants, 
especially  those  growing  in  the  drier  regions,  by  which  trans- 


MOVEMENTS   OF   WATER  IN   THE  PLANT.  2$ 

piration  may  be  lessened  or  controlled,  are  very  numerous.  One 
of  the  most  effective  is  a  covering  of  branching  hairs.  Much  the 
greater  part  of  the  water  thrown  off  by  the  plant  is  transpired 
from  the  thin-walled  cells  in  the  interior  of  the  leaf  into  the  inter- 

FIG.  23. 


Air-chamber  and  opening  of  Marchantia  polymorpha  :  magnified  300  times. 

(Kerner.) 

cellular  spaces  which  communicate  with  the  open  air  through 
the  stomata.  The  stomata  (Figs.  22  and  23)  are  openings  in 
the  epidermis,  which  are  controlled  by  guard-cells.  When, 
more  water  is  transpired  from  the  leaf  than  is  furnished  by  the 
roots,  the  guard-cells  become  flaccid,  and  the  walls  are  thick- 
ened in  such  manner  that  in  this  condition  these  cells  change, 
their  form  and  close  the  openings  of  the  stomata  entirely. 
When  the  necessary  water-supply  is  at  hand  the  guard-cells  are- 
turgid  and  the  stoma  remains  open.  The  action  of  the  guard- 
cells  is  also  influenced  by  light,  wind,  and  other  factors. 
Transpiration  is  increased  by  heat,  light,  dryness,  high  pres- 
sure,  and  movements  of  the  air,  and  lessened  by  the  opposite 
conditions. 

EXPERIMENT  29. 

INFLUENCE   OF    HUMIDITY    ON   THE   AMOUNT   OF    TRANSPIRATION. 

Place  a  well-leaved  Begonia  grown  in  a  pot,  on  one  pan  of  a  drug- 
gist's balance.  Cover  the  soil  by  means  of  two  glass  plates,  or  tie  a 
piece  of  oiled  cloth  around  the  entire  pot,  to  prevent  evaporation. 


26 


EXPERIMENTAL   PLANT  PHYSIOLOGY. 


By  means  of  weights  on  the  other  pan  equalize  the  balance.  In  an 
hour  it  will  be  noted  that  the  end  of  the  scale  holding  the  plant  has 
risen.  Take  weights  from  the  other  pan  until  the  equipoise  is  restored. 
The  amount  of  the  weights  taken  off  will  represent  water  transpired 
by  the  plant.  After  balancing  cover  the  plant  by  means  of  a  bell- 

FIG.  24. 


Estimation  of  amount  of  transpiration  by  weighing.     (After  Oels.) 

jar.  In  an  hour  remove  the  bell-jar,  quickly  wipe  from  the  pan  the 
water  which  may  have  condensed  and  run  down  the  sides  of  the 
bell-jar,  and  again  take  off  weights  to  balance.  The  amount  lost 
will  be  less  than  before.  The  air  in  the  bell-jar  soon  becomes  satu- 
rated with  water  and  checks  transpiration.  (Fig.  24.) 

EXPERIMENT  30. 

INFLUENCE   OF    EPIDERMIS    ON   TRANSPIRATION. 

Select  two  Apples  and  two  Potatoes  of  equal  size.  Peel  one  of 
of  each.  Weigh  and  set  aside  for  three  hours.  Again  weigh.  It 
will  be  seen  that  a  waxy  or  corky  epidermis  retards  transpiration 
very  efficiently. 

20.  Wilting. — If  the  amount  of  water  transpired  exceeds 
that  absorbed  by  the  roots,  wilting  results.  This  may  occur 
from  the  destruction  of  the  root-hairs  or  from  an  insufficient 
supply  of  water  in  the  soil.  In  the  transplantation  of  trees  the 
branches  are  trimmed  in  order  that  the  transpiring  surface 
may  be  reduced  in  proportion  to  the  absorbing  surface.  The 
latter — in  the  root-hairs — is  nearly  all  destroyed  by  transplan- 


MOVEMENTS   OF    WATER   IN    THE  PLANT.  2*J 

tation.     The  turgor  of  a  wilted  plant  may  be  restored  either 
by  watering  the  soil  or  checking  transpiration. 

EXPERIMENT  31. 

RESTORATION    OF   A   WILTED    PLANT   BY   CHECKING   TRANSPIRATION. 

A  plant  if  not  too  badly  wilted  will  revive  if  placed  under  a  bell- 
jar  or  if  transpiration  is  checked  by  other  means. 

21.  Guttation. — If  the  amount  of  water  absorbed  by  the 
roots  is  in  excess  of  that  transpired  by  the  leaves,  it  will  exude 
through  rifts  in  the  epidermis,  or  the  water-pores,  in  the  form  of 
drops.    This  process  is  termed  guttation.     It  may  be  observed 
in  plants  at  the  end  of  a  warm  day.     The  air  cools  quickly, 
and  its  relative  humidity  is  increased  while  the  roots  absorb 
the  same  amount  of  water  from  the  soil,  which   retains   its 
warmth  for  a  longer  time. 

EXPERIMENT  32. 

GUTTATION    PRODUCED   BY    CHECKED    TRANSPIRATION. 

Cover  a  plant  such  as  Corn,  Wheat,  or  Pea  with  a  bell-jar  and 
place  in  sunlight.  Note  the  drops  of  water  on  the  leaves  after  an 
hour  or  two. 

22.  Attraction  of  Soil  for  Water. — Plants  cannot   either   by 
the  force  of  diffusion   or  of  transpiration   absorb  all  of  the 
water  in  the  soil.     Absorption  finally  reaches  a  limit  beyond 
which  the  attraction  of  the  soil-particles  for  water  is  stronger 
than  the  combined  force  of  diffusion  and  evaporation  in  the 

plant. 

EXPERIMENT  33. 

AMOUNT   OF   WATER   IN   THE    SOIL   WHICH    CANNOT   BE   ABSORBED. 

Grow  a  plant  (Bean)  in  a  pot  filled  with  rich  garden  soil.  As 
soon  as  the  primordial  leaves  have  developed,  place  in  a  room  ex- 
posed to  direct  sunlight,  and  allow  it  to  remain  without  watering  until 
it  wilts.  Now  take  a  sample  of  a  few  grams  of  the  soil  which  has 
been  penetrated  by  the  roots,  and  dry  at  100°  C.  for  an  hour.  Weigh. 
It  is  demonstrated  that  the  soil  contained  a  large  percentage  of 
water  which  the  plant  could  not  obtain  to  replace  its  evaporation. 


28  EXPERIMENTAL  PLANT  PHYSIOLOGY. 

23.  Uses  of  Transpiration. — In   the  economy  of  the  plant 
transpiration  is  of  the  greatest  importance.     Water  and  dis- 
solved nutrient  salts  are  carried  to  the  leaves  by  the  transpira- 
tion stream.     The  greater  part  of  the  water  evaporates,  and 
the  remainder,  with  the  salts,  is  formed  into  compounds  useful 
to  the  plant.     In  the  leaves  the  simple  "  power  of  selection  " 
operates  as  in  the  roots,  and  only  the  salts  they  can  use  are 
carried  to  them  in  quantity.     It  is  probable  that  transpiration 
serves  other  uses  which  are  not  yet  clearly  understood.     The 
suggestion  has  been  made  that  it  equalizes  changes  of  tempera- 
ture in  the  plant. 

24.  Ascent  of  Sap. — The  forces  concerned  in  carrying  water 
from  the  roots  to  the  leaves  are  root-pressure,  capillary  action 
of  the  wood-cells,  imbibition,  diffusion,  expansion  and  contrac- 
tion of   the  air-bubbles  in  the  wood-cells,  transpiration,  and 
osmotic  action  of   the  protoplasm  of   the  wood-parenchyma 
cells. 

In  small  herbaceous  plants  root-pressure  is  almost  always 
present,  and  it  acts  with  a  force  sufficient  (see  paragraph  15)  to 
drive  water  to  the  leaves.  In  plants  of  this  character  the  suc- 
tion exerted  by  transpiration  is  also  sufficient  to  carry  water 
upward  to  the  desired  height.  (See  Experiment  26.)  The 
other  factors  mentioned  are  of  minor  importance  in  such 
plants. 

In  trees,  however,  which  may  attain  a  height  of  10  to  150 
meters,  the  manner  in  which  the  necessary  water-supply  is 
carried  to  the  leaves  becomes  a  question  of  great  complexity. 
Root-pressure  is  present  in  trees  only  during  a  limited  period 
at  the  beginning  of  the  growing  season  and  is  almost  entirely 
absent  in  summer  when  the  greatest  amount  of  water  is  used. 
Hence  it  cannot  bear  a  very  important  part  in  the  ascent  of 
sap.  The  transpiration  of  water  from  the  leaves  creates  a 
vacuum  in  the  stem  below,  as  has  been  demonstrated  in 


MOVEMENTS   OF   WATER   IN   THE  PLANT.  29 

Experiment  26.  (See  also  Experiment  35.)  The  suction  thus 
caused  would  not  raise  water  higher  than  a  suction-pump 
(about  10  meters).  The  water,  however,  is  not  in  a  continuous 
tube  like  the  cylinder  of  a  pump.  The  rectangular  wood-cells 
are  in  .the  form  of  a  series  of  chains.  The  water  in  each  cell 
is  separated  from  that  of  the  neighboring  cells  by  a  thin  mem- 
brane which  promotes  osmose.  Water  is  transpired  from  the 
topmost  cells  of  these  chains,  the  cell-sap  becomes  concen- 
trated and  draws  water  from  the  cells  beneath,  and  they  in  turn 
from  those  beneath  them.  There  is  thus  formed  a  series  of 
osmometers  extending  from  the  leaves  to  the  roots,  and 
capable  of  lifting  water  to  any  height. 

In  passing  from  the  lower  to  the  upper  end  of  the  narrow 
wood-cells,  the  ascent  of  sap  is  greatly  retarded  by  capillary 
friction.  On  the  other  hand,  the  cavity  of  a  wood-cell  con- 
tains a  bubble  of  gas,  which  by  its  expansion  and  contraction 
aids  in  forcing  the  sap  upward.  Further,  imbibition  by  the 
cell-wall  allows  the  passage  from  one  part  of  the  plant  to 
another  of  a  small  amount  of  water  which  does  not  enter  the 
cell-cavities. 

It  is  difficult  to  account  for  the  rapidity  of  sap-movements 
by  the  action  of  these  physical  forces  alone.  Some  investiga- 
tions tend  to  show  that  the  protoplasm  of  the  wood-parenchyma 
has  a  rhythmic  osmotic  attraction  for  water.  Some  such  force 
is  necessary  to  account  for  all  features  of  sap-movement  in 
trees. 

EXPERIMENT  34. 

AMOUNT  OF    WATER    FORCED    UPWARD    BY    ROOT-PRESSURE    COMPARED    WITH 
THAT  TRANSPIRED   BY   THE   LEAVES   OF  AN   HERBACEOUS    PLANT. 

With  a  sharp  knife  cut  off  a  strongly  growing  Sunflower  plant 
near  the  ground.  Fasten  the  upper  part  with  its  cut  end  in  a 
measuring-cylinder  containing  water.  To  the  stump  (lower  part) 


30  EXPERIMENTAL   PLANT  PHYSIOLOGY. 

fasten  by  means  of  rubber  tubing  a  tube  bent  twice  at  right  angles. 
Insert  the  free  end  of  the  tube  in  a  test-tube.  The  water  thrown 
out  and  through  this  tube  by  root-pressure  will  be  collected  in  the 
test-tube,  and  its  volume  can  be  compared  with  the  amount  drawn 
out  of  the  measuring-cylinder  by  the  transpiration  of  the  other  part 
of  the  plant  (Fig.  25). 

FIG.  25. 


^l^^^^^^^Z'" 


Comparison  of  root-pressure  and  transpiration.     (After  Oels.) 


MOVEMENTS   OF   WATER  IN   THE  PLANT. 


FIG.  26. 


EXPERIMENT    35. 

NEGATIVE  PRESSURE. 

In  September  bore  a  small  hole  6  cm. 
in  depth  in  the  trunk  of  a  small  Birch, 
and  fit  into  the  opening  a  glass  tube  a 
meter  in  length  which  has  been  bent 
once  at  right  angles.  Make  the  fitting 
"  air-tight  "  by  means  of  wax.  Place 
the  lower  end  of  the  tube  in  a  dish  of 
mercury.  In  a  day  or  two  the  mercury 
will  rise  in  the  tube  to  a  varying  height. 
The  rapid  transpiration  from  the  leaves 
has  withdrawn  so  much  water  from  the 
trunk  of  the  tree  that  a  partial  vacuum 
is  formed.  (Fig.  26.)  This  may  also 
be  demonstrated  as  follows  (Fig  27)  • 


Negative  pressure  in  Birch 
stem.     (After  Oels.) 


Negative  pressure  in  shoot  of  Lonicera.     (Detmer.) 


.32  EXPERIMENTAL   PLANT  PHYSIOLOGY. 

Cut  off  a  shoot  of  some  woody  plant  with  tender  leaves  (Loni- 
cera),  and  place  the  lower  end  in  a  vessel  of  water.  Now  cut  off 
the  top  and  fasten  to  the  end  of  the  shoot,  by  means  of  a  piece  of 
rubber  tubing,  a  glass  tube  bent  twice  at  right  angles.  Place  the  end 
of  the  perpendicular  long  arm  in  a  vessel  of  mercury.  In  a  short 
time  the  fluid  ascends  in  the  tube. 

EXPERIMENT  36. 

RESTORATION   OF   SAP-CURRENT. 

Fix  an  excised  shoot  of  Coleus  or  Helianthus  (Sunflower)  by 
means  of  a  rubber  stopper  in  one  arm  of  a  U  tube  and  fill  with  water. 

Its  power  of  conduction  has  been  de- 
stroyed and  it  wilts  (See  Experiment  28). 
Now  pour  mercury  into  the  free  arm  of 
the  tube.  The  turgor  is  restored,  and  is 
retained  until  the  mercury  is  higher 
under  the  plant  than  in  the  other  arm. 

EXPERIMENT  37. 

RATE   OF   ASCENT   OF   SAP. 

Water  copiously  the  soil  in  which  2 
herbaceous  plant  i  meter  in  height  is 
growing,  with  a  solution  of  lithium  ni- 
trate. In  an  hour  cut  a  portion  from 
the  tip  and  at  successive  intervals 
toward  the  root.  Burn  these  pieces 
in  the  flame  of  an  alcohol-lamp  or  Bun- 
sen  burner,  and  by  the  characteristic 
red  flame  of  lithium  ascertain  to  what 

Restoration  of  sap-current,   height  the  lithium  has  ascended  in  the 
(Sachs.)  stem< 

EXPERIMENT  38. 

MOVEMENT   OF    FLUIDS    IN   CONTINUOUS    VESSELS. 

Cut  away  the  stem  of  a  Euphorbia  (Spurge),  Sonchus  (Wild 
Lettuce),  or  Asclepias  (Milkweed).  The  milky  juice  exudes  rapidly 
from  both  the  upper  and  lower  cut  surfaces  in  a  manner  indicative 
of  pressure.  Cut  away  the  stem  of  a  Gourd  or  Pumpkin  and  note 
the  large  drops  of  slime  which  must  have  been  forced  from  some 
distance,  since  that  amount  would  not  be  found  in  the  cells  of  the 
part  cut  across. 


MOVEMENTS   OF   WATER  IN   THE  PLANT. 


33 


25.  Path  of  Sap  Movements. — The  plant  takes  up  water  and 
mineral  salts  from  the  soil,  and  forms  foods  from  carbon  diox- 
ide in  the  leaves.  These  substances  must  pass  from  the  roots 
upward  and  from  the  leaf  downward  to  be  of  use  to  the  plant. 
The  ascending  stream  moves  upward  through  the  woody  part 
(xylem)  of  the  stem.  In  trees  the  greater  amount  passes 
FIG.  29.  FIG.  30. 


Cross-section  of  portion  of  shoot  of  Cross-section  of  portion  of  stem 
Sambucus  nigra  (Elder)  magnified  15  of  Sambucus  nigra  (Elder) 
times.  (After  Oels.)  ^,  epidermis  ;  magnified  150  times.  (After 
k,  cork  ;  rp,  parenchyma  and  scleren-  Oels.)  rp,  phloem  paren- 
chyma ;  c,  cambium  ;  ^,  wood  ;  #//,  chyma  ;  sc,  sclerenchyma  ;  c, 
pith.  cambium  ;  h,  wood  ;  m,  me- 
dullary rays. 

through  five  or  six  of  the  recently-formed  annual  rings,  as  may 
be  seen  in  trees  with  hollowed  trunks  which  sustain  tops  of  nor- 
mal size.  The  descending  current  passes  through  the  soft  inner 
bark,  the  phloem.  The  descending  current  moves  very  slowly, 
and  is  carried  on  principally  by  diffusion  (Figs.  29  and  30). 
EXPERIMENT  39. 

UPWARD     PATH     OF     SAP. 

Remove  a  ring  of  the  bark  and  soft  wood  from  any  young 
tree  or  woody  shoot  a  few  centimeters  above  the  ground  by  means 
of  a  sharp  knife.  The  shoot  shows  no  disturbance  for  a  time  vary- 
ing from  a  few  weeks  to  many  months,  when  the  roots  become  starved 
from  lack  of  food  usually  supplied  by  the  leaves  and  perish. 


34 


EXPERIMENTAL   PLANT  PHYSIOLOGY. 


EXPERIMENT  40. 

DOWNWARD   PATH   OF   SAP. 

In   the  same  manner  as  above  girdle  a  Willow  branch    i   to 
cm.  in   diameter  by  removing  a  ring  of  bark  near  the  lower  end. 
FIG.   31. 


S 


Place  upright  with  the  lower  end  sub- 
merged in  water.  The  buds  develop  in  a 
normal  manner  while  roots  are  formed  on  the 
lower  end,  but  only  above  the  girdling  ring. 
Since  the  phloem  is  removed,  the  food-mate- 
rial necessary  for  the  formation  of  the  roots 
cannot  pass  the  ring.  (Fig.  31.) 

EXPERIMENT  41. 

DEMONSTRATION   OF   PATH   OF   SAP   BY   COLORED 
FLUID. 


Girdled  shoot  of  Sambu- 
cus.     (After  Oels.) 


Cut  off  a  semi-transparent  stem  of  Impa- 
tiens  (Touch-me-not),  and  place  the  lower  end 
in  a  water  solution  of  some  aniline  color.  In  an  hour  note  that  the 
colored  fluid  has  ascended  in  the  woody  fibres  in  the  soft  stem. 
Repeat,  using  a  stalk  of  a  young  Corn  plant.  Allow  it  to  stand  in 
the  solution  24  hours,  then  dissect  and  determine  the  path  of  the 
fluid. 


CHAPTER  HI. 

ABSOPTION   OF   GASES. 

26.  Gases  used   by   the  Plant. — Of  the    gaseous   elements 
which  enter  into  the  food  of  plants,  hydrogen  is  taken  up  in 
the  form  of  water  or  ammonia  by  ordinary  green  plants,  while 
it  forms  a  large  proportion  of  the  complex  substances  which 
are  used  by  parasitic  and  saprophytic  plants.     Oxygen  is  ob- 
tained from  the  air  in  a  free  state,  and  in  combination  in  the 
form  of  water,  carbon  dioxide,  and  the  mineral  salts.    Nitrogen 
is  derived  chiefly  from  compounds  in  the  soil.     Leguminous 
plants  and  many  groups  of  the  lower  forms  are  able  to  take 
up  this  element  directly  from  the  atmosphere.     The  greater 
part  of  it  used  by  the  higher  plants  has  been  fixed  in  the  soil 
by  the  action  of  Bacteria  and  related  forms.     At  the  present 
time  the  power  of  the  various  groups  of  plants  to  take  up  free 
nitrogen  is  not  clearly  defined.     Carbon  is  obtained  by  plants 
which  do  not  contain  chlorophyll  from  the  complex  compounds 
which  they  use  as  food.     This  is  true  of  all  plants  which  use 
complex  foods.     Green  plants,  however,  obtain  their  carbon 
supply  from  the  carbon  dioxide  of  the  air.     (§  29.) 

27.  Diffusion  of  Gases. — If  two  gases  that  will  mix  are  sepa- 
rated by  a  membrane,  they  will  pass  through  the  membrane  by 
osmose  in  the  same  manner  as  liquids.     The  air  is  a  mixture 
of  77.95  parts  of  nitrogen,  20.61  parts  of  oxygen,  1.40  parts 

35 


36  EXPERIMENTAL  PLANT  PHYSIOLOGY. 

of  aqueous  vapor,  and  .04  part  of  carbon  dioxide.  These 
gases  are  in  different  proportions  in  the  plant  and  conse- 
quently a  constant  diffusion  through  the  outer  membrane 
takes  place.  Some  cells  containing  substances  which  have  a 
high  osmotic  equivalent  for  oxygen  absorb  it  from  the  air. 
In  like  manner  cells  containing  chlorophyll  take  up  carbon 
dioxide  during  the  daytime.  Gases  will  readily  diffuse 
through  a  membrane,  yet  cannot  be  forced  through  it  by 
pressure. 

EXPERIMENT  42. 

DIFFUSION   OF   GAS   THROUGH    EPIDERMIS. 

Smooth  one  end  of  a  glass  tube  with  an  internal  diameter  of  .5 
cm.  and  a  length  of  30  cm.  in  a  flame.  Select  a  smooth  and  perfect 
grape.  Take  off  the  skin  and  clean  the  pulp  from  the  inside. 
Place  over  the  end  of  the  tube,  bringing  the  edges  down  and  fasten- 
ing closely  to  the  tube  by  a  small  cord.  (Fig.  32.)  With  sealing- 
wax  secure  the  edges  to  the  glass  in  such  a  manner  as  to  be  "  air- 
tight." Test  by  placing  in  water  and  forcing  air  in  at  the  other  end. 
If  no  bubbles  escape,  fill  the  tube  with  water  and  invert  in  a  vessel 
of  mercury.  Displace  the  water  with  carbon  dioxide  and  note  the 
height  of  the  mercury  column  daily  for  a  month.  By  the  diffusion 
of  the  carbon  dioxide  through  the  membrane  the  column  of  mercury 
may  be  raised  as  high  as  26  cm. 

Remark. — In  inverting  the  tube  when  full  of  water  no  air  must  be 
allowed  to  gain  entrance.  To  obtain  carbon  dioxide  use  the  apparatus  de- 
scribed in  Experiment  57.  Marble  and  hydrochloric  acid  should  be  used 
instead  of  zinc  and  sulphuric  acid,  as  there  described. 

28.  Absorption  of  Hydrogen,  Oxygen,  and  Nitrogen.  —  The 
absorption  of  hydrogen,  oxygen,  and  nitrogen,  and  their 
synthesis  into  food  are  so  closely  connected  with  other 
complex  metabolic  processes  that  a  consideration  of  the 
separate  action  in  each  case  is  somewhat  difficult.  The 
manner  in  which  carbon  is  obtained  and  used  is,  however, 
a  fairly  distinct  process. 


ABSORPTION  OF  GASES. 


37 


29.  Photosynthesis.  —  Green  plants  absorb  carbon  dioxide 
from  the   air   either   through  FIG.  32. 

the  epidermis  or  the  stomata. 
Carbon  dioxide  is  composed 
of  one  part  of  carbon  and  two 
parts  of  oxygen.  The  proto- 
plasm which  forms  the  mass 
of  the  green  color  bodies 
(chlorophyll  bodies)  in  the 
cells  has  the  power,  when  it 
receives  the  sunlight,  of  sep- 
arating one  part  of  the  oxy- 
gen which  is  thrown  off  as 
a  free  gas,  while  the  carbon 
monoxide  which  remains  is 
combined  with  the  water 
present  to  form  a  compound 
of  carbon,  hydrogen,  and 
oxygen  from  which  sugar  is 
ultimately  derived.  The  en- 
tire process  may  be  desig- 
nated photosynthesis.  No 
life  is  imaginable  without 
photosynthesis.  All  organ- 

.  Diffusion    of  gas  through  epidermis. 

isms,    plants,     and     animals       /,  level  of  mercury  column  20  days 


alike   are   ultimately   depend- 

ent     upon     the     products     of       centimeter-scale. 

this  process  for  their  carbon  compounds. 
EXPERIMENT  43. 

THE   ACTION   OF   LIGHT   IS    NECESSARY   FOR    PHOTOSYNTHESIS. 

Weigh  4  seeds  of  Corn,  germinate  and  grow  in  nutrient  solution. 
Place  2  of  the  seedlings  in  a  dark  chamber  and  the  remaining  2  in 
the  sunlight.  In  three  weeks  take  the  plants  from  the  solutions,  dry 


EXPERIMENTAL   PLANT  PHYSIOLOGY. 


in  the  air  for  several  days  and  weigh.  Those  in  darkness  will  have 
lost,  while  those  in  light  will  have  gained  weight  since  they  were  able 
to  form  food  from  carbon  dioxide  of  the  air  and  water. 

EXPERIMENT  44. 

OXYGEN   IS    GIVEN   OFF   DURING   PHOTOSYNTHESIS. 

Fill  a  funnel  of  medium  size  with  green  shoots  of  Elodea  8  to  10 
centimeters  in  length.  Immerse  the  funnel,  inverted,  in  a  wide  dish 
filled  with  spring-water,  and  over  the  small  end  of  the  funnel  place 
a  test-tube  filled  with  water.  Set  in  the  sunlight.  In  a  short  time 


FIG.  33- 


FIG.  34. 


Apparatus  to  show  excretion  ot 
oxygen  by  Elodea.     (Detmer.) 


Action  of  light  on  Elodea. 
(After  Dels.) 


gas  can  be  seen,  collected  in  the  upper  part  of  the  tube,  which  tested 
with  a  glowing  splinter  is  proved  to  be  oxygen. 

EXPERIMENT  45. 

THE  AMOUNT   OF   PHOTOSYNTHESIS    AND   OF   OXYGEN    GIVEN   OFF   DEPENDS   ON 
THE   INTENSITY    OF   THE    LIGHT. 

Fasten  a  shoot  of  Elodea  about  10  centimeters  long  to  a  glass 
rod  and  immerse  in  spring-water  or  water  containing  carbon  dioxide, 
so  that  the  cut  end  is  higher  than  the  other.  Set  the  apparatus  in 
direct  sunlight,  and  immediately  a  stream  of  gas-bubbles — oxygen — 
begins  to  pour  from  the  cut  end  of  the  shoot.  (Fig.  34.)  In  diffused 


ABSORPTION  OF  GASES. 


39 


light,  or  in  light  the  intensity  of  which  is  reduced  by  means  of  one 
or  more  plates   of  ground  glass  (Fig.  35),  the  number  of  bubbles 

FIG.  35- 


Box  blackened  on  the  inside.     (After  Oels.)     a  a,  ground-glass  plates  ;  l>, 

shoot  of  Elodea. 

given  off  decreases,  so  that  the  dependence  of  photosynthesis  upon 
light  can  be  seen  directly. 

30.  Physical  Properties  of  Chlorophyll. — Chlorophyll  is  a  sub- 
stance of  extremely  complex  and  unstable  constitution.  It  is 
generally  found  in  certain  definite  masses  of  protoplasm  in  the 
cell,  although  in  some  plants  it  appears  uniformfy  diffused 
throughout.  Its  presence  is  sometimes  masked  by  other  color- 
ing matter,  as  in  the  Red  Sea-weeds  and  colored  leaves  of  the 
foliage  plants  of  the  garden.  Some  of  the  autumnal  tints  of 
leaves  are  due  to  coloring  substances  resulting  from  the  oxida- 
tion of  chlorophyll.  The  spectrum  of  sunlight  which  has 
passed  through  a  solution  of  chlorophyll  in  alcohol  shows 
several  dark  bands.  The  portions  of  light  thus  absorbed  are 
converted  into  heat  and  other  forms  of  energy  needed  for 
photosynthesis  and  other  processes.  If  different  portions  of  the 
spectrum  are  allowed  to  act  on  a  plant,  the  relative  amount  of 


EXPERIMEN TA  L   PLA N T-PH YSIOL OGY. 


photosynthesis  promoted  by  each  can  be  demonstrated.     The 
red  rays  are  principally  active  in  photosynthesis. 

31.  Division  of  the  Spectrum. — It  is  found  that  a  watery  solu- 
tion of  potassium  bichromate  transmits  only  the  red,  orange, 
and  yellow  rays  of  light,  and  that  an  ammoniacal  solution  of 
copper  exide  transmits  only  the  blue  and  violet  rays.  Unless 
carefully  compounded,  however,  the  latter  solution  will  also 
allow  the  passage  of  some  of  the  red  and  yellow  rays. 

EXPERIMENT  46. 

PORTION    OF   THE    SPECTRUM   ACTIVE   IN    PHOTOSYNTHESIS. 

Make  a  solution  of  potassium  bichromate  in  water.  To  obtain 
the  ammonia-copper-oxide  solution,  add  ammonium  hydrate  to  a 
solution  of  copper  sulphate  in  water  as  long  as  the  forming  precipi- 
tate is  redissolved.  Fill  a  double-walled  bell-jar  with  each  solution. 


FIG.  36. 


FIG.  37. 


Double-walled   bell-jar. 
(After  Oels.) 


Apparatus  to  replace  double 
walled  bell-jar.  (After  Oels.) 
k,  solution  of  potassium  bi- 
chromate or  copper  oxide;  w, 
water  ;  />,  pasteboard  cover. 


Prepare  three  shoots  of  Elodea  as 

in  Experiment   45.      Place   one   in 

sunlight  and  one  under  each  bell-jar.     Bubbles  of  oxygen  are  given 

off  from  the  shoots  in  open  sunlight  and  under  the  red  bell-jar,  but 

none  from  the  one  under  the  blue  bell-jar.     (Fig.  36.) 


ABSORPTION  OF  GASES.  41 

If  the  double-walled  bell-jars  are  not  at  hand,  each  may  be  re- 
placed   by  two  glass  cylinders,  one  so  much   larger        FIG.  38. 
than  the  other  that  when  the  smaller  is  fastened  in- 
side the  other  by  means  of  a  cork,  a  space  of  about 
i   to  2  cm.   remains  between  them.     Fill  this  space 
with   the  proper  solution,  and  the  inner  vessel  with 
water    containing   carbon    dioxide,  and  place  in  the 
latter  the  plant-shoots.     To  cut  off  the  perpendicu- 
lar  rays    cover   the    apparatus   with  a   loosely-fitting 
cardboard   cover.      (Fig.  37.)      The   pasteboard  box 
shown  in  Fig.  35  can  also  be  used  if  instead  of  the 
ground-glass   plates   (a  a]   parallel-walled   glass    cells 
filled  with  the  absorption  fluid  are  used.    With  colored  Apparatus     to 
glass  plates  only  an  approximately  pure  light  can  be     JjJJSSSon 
obtained.     If  the  box  is  used,  it  will  be  found  most     of     oxygen, 
convenient  to  place  the  shoot  in  an  inverted  test-tube     (Mangln-) 
filled  with  water  as  in  Fig.  38.     The  amount  of  gas  can  be  measured 
directly  in  the  tube. 

32.  Product  of  Photosynthesis. — The  product  of  photosyn- 
thesis is  probably  some  soluble  carbohydrate  such  as  glucose. 
As  soon  as  enough  of  this  substance  has  been  formed  to  meet 
the  immediate  needs  of  the  plant  the  remainder  is  converted 
into  starch.  If  the  plant  is  placed  in  darkness  or  under  any 
condition  in  which  it  cannot  carry  on  photosynthesis,  as  soon 
as  the  glucose  in  the  cells  is  consumed  the  starch  is  recon- 
verted into  glucose  or  some  form  of  sugar  and  assimilated. 
The  amount  of  starch  present  in  a  plant  may  be  taken  as 
an  indirect  indication  of  the  amount  of  photosynthesis. 

EXPERIMENT  47. 

MACROCHEMICAL   TEST    FOR    STARCH. 

Boil  a  few  leaves  of  Bean,  Tomato,  or  Tropaeolum  for  a  few 
minutes  to  kill  the  protoplasm  and  swell  the  starch-grains  present. 
Place  in  warm  alcohol  until  the  chlorophyll  is  dissolved.  Bring 
the  leaves  into  an  alcoholic  solution  of  iodine  for  a  half-hour. 
The  leaves  will  be  colored  a  dark  blue  if  they  contain  starch. 


42  EXPERIMENTAL   PLANT  PHYSIOLOGY. 

EXPERIMENT  48. 

MICROCHEMICAL   TEST   FOR   STARCH. 

Decolorize  some  filaments  of  Spirogyra,  or  leaves  of  Funaria  as 
above,  and  place  on  a  glass  slide  in  a  drop  of  chloral  hydrate 
(chloral  hydrate  5  parts,  water  2  parts).  Add  a  drop  of  a  solution 
of  iodine  in  iodide  of  potassium,  and  examine  with  the  microscope. 
A  thin  section  of  large  leaves  may  be  examined  in  this  manner. 

EXPERIMENT   49. 

STARCH   AS   AN   INDICATION   OF   PHOTOSYNTHESIS. 

Place  some  Spirogyra  or  Vaucheria  in  a  dark  chamber  for  24 
hours.  Test  some  of  the  filaments  for  starch.  It  will  be  found 
absent.  Set  the  vessel  containing  the  filaments  in  the  sunlight  for 
a  few  minutes.  Examine  a  second  lot.  Starch  will  be  found 
present. 

EXPERIMENT  50. 

FORMATION   OF   STARCH    FROM    SUGAR. 

Deprive  a  Geranium  plant  of  starch  by  placing  in  a  dark  cham- 
ber for  24  hours  or  longer.  Test  for  starch,  and  if  none  is  present 
cut  off  a  leaf  and  place  it  in  a  20$  solution  of  sugar  for  a  week  in  a 
dark  chamber.  Test  for  starch.  The  protoplasm  of  the  leaf  has 
used  the  sugar  as  food  and  has  also  converted  a  portion  of  it  into 
starch. 


CHAPTER  IV. 

RESPIRATION  AND  OTHER  FORMS  OF   METABOLISM. 

33.  Nature  of  Metabolism. — By  various  processes,  of  which 
photosynthesis  is  an    important  example,  a  large  number  of 
complex  substances  are  formed  in  the  plant.     The  synthesis  of 
complex   compounds    from   those  of   simpler  composition   is 
termed  constructive  metabolism.    In  this  process  a  portion  of  the 
oxygen  in  the  simple  compounds  is  liberated.     Thus  in  photo- 
synthesis water  and  carbon  dioxide,  both  containing  oxygen, 
are  combined  and  a  portion  of  the  oxygen  is  set  free.     This 
liberation  of  oxygen  makes  constructive  metabolism  what  is 
known  in  chemistry  as  a  reducing  process.     On  the  other  hand, 
when  the  complex  foods  thus  formed  are  used  by  the  plant, 
oxygen  is  taken  up  and  the  complex  substances  are  resolved 
into  others  of  simpler  composition.     This  is  known  as  destruc- 
tive metabolism.    Since  oxygen  is  absorbed  in  destructive  meta- 
bolism, it  is  essentially  an  oxidizing  process.     One  of  its  most 
important  forms  is  respiration. 

34.  Respiration. — In  respiration,  which  is  directly  opposite 
in  character  to  photosynthesis,  oxygen   is  absorbed,  carbon 
dioxide  given  off,  and  energy  liberated  in  the  forms  of  heat 
and  electricity.     The  oxygen  needed  is  largely  obtained  from 
the  air,  although   in  some  instances  it  is  derived  from  com- 
pounds in  the  plant  which  contain  a  large  proportion  of  it. 
The    extraction    of   oxygen    from    one   substance  within    the 

43 


44 


EXPERIMENTAL   PLANT  PHYSIOLOGY. 


plant  for  the  oxidation  of  another  is  termed  intramolecular 
respiration.  This  form  of  respiration  is  carried  on  to  some 
extent  by  all  plants,  but  is  characteristic  of  the  germination  of 
oily  seeds  and  of  Yeast,  Bacteria,  etc. 

Respiration  is  essentially  the  same  process  in  both  plants 
and  animals,  but  while  the  former  breathe  and  give  off  carbon 
dioxide  constantly  from  all  parts  of  their  bodies,  the  latter, 
in  the  highly-developed  forms,  breathe  rhythmically  and  for 
the  greater  part  by  means  of  organs  especially  adapted  for  the 
purpose.  Yet  there  are  instances  among  both  plants  and 
animals  where  the  respiratory  processes  are  suspended  or  re- 
duced for  a  period  of  varying  length. 

35.  Absorption  of  Oxygen  and  Excretion  of  Carbon  Dioxide. — 
The  amount  of  carbon  dioxide  given  off  in  respiration  is 
FIG.  39.  FIG.  40. 


Liberation  of  carbon  dioxide  by 
respiration.  (Mangin.)  a, 
baryta-water. 

approximately  equal  to  the 
oxygen  taken  up.  The 
proportion  of  the  two  sub- 
stances varies  with  the  tem- 
perature and  other  condi- 
tions, and  in  the  different  organs.  De  Saussure  found  that  i 
gram  of  seed  of  Hemp  absorbed  19.7  cc.  of  oxygen  and  exhaled 


Cylinder  containing  germi- 
nating Peas.     (Sachs.) 


RESPIRATION  AND    OTHER  FORMS   OF  METABOLISM.    45 

13.26  cc.  of  carbon  dioxide  in  the  same  time,  and  I  gram  of  seed 
of  Madia  absorbed  15.83  cc.  of  oxygen,  while  it  exhaled  11.94 
cc.  of  carbon  dioxide.  Young  growing  plants  will  exhale  an 
amount  of  oxygen  equal  to  their  own  volume  in  24  to  36 
hours. 

EXPERIMENT  51. 

EXCRETION   OF   CARBON    DIOXIDE   BY   LEAVES. 

Provide  a  ground-glass  bell-jar  and  plate.  Under  the  bell-jar 
place  a  well-leaved  plant  grown  in  a  pot,  and  a  vessel  containing 
lime-  or  baryta-water  ;  place  the  apparatus  in  darkness.  After  a 
short  time  a  film  of  carbonate  can  be  seen  on  the  surface  of  the  fluid 
which,  if  allowed  to  remain  longer,  collects  as  chalk  (or  baryta).  As 
a  control  experiment,  set  up  the  same  apparatus  without  the  plant. 
The  lime-  or  baryta-water  is  scarcely  affected.  (Fig.  39.) 

EXPERIMENT  52. 

EXCRETION   OF   CARBON   DIOXIDE   BY    GERMINATING   SEEDS. 

Fill  a  glass  jar  of  i  liter  capacity  one-third  full  of  Peas  which 
have  lain  a  day  in  water.  Cover  tightly.  After  12  or  14  hours  a 
light  thrust  in  is  extinguished,  showing  the  lack  of  oxygen,  and  a 
vessel  containing  lime-  or  baryta-water  placed  inside  demonstrates 
the  presence  of  carbon  dioxide.  Instead  of  Peas,  developing  heads 
of  a  Composite  or  some  large  Fungus  can  be  used.  (Fig.  40.) 

36.  Liberation  of  Heat. — In  very  strong  respiration,  as  in 
the  development  of  flower-heads  of  the  Compositae,  flower-tubes 
of  the  Aroids,  and  germinating  seeds,  enough  heat  is  liberated 
in  the  combustion  of  the  carbon  compounds  of  the  plant  to 
be  easily  detected  by  the  thermometer.  Sachs  observed  in 
100  to  200  germinating  Peas  a  rise  in  temperature  of  1.5°  C. 
(Fig.  41.) 

EXPERIMENT  53. 

HEAT   LIBERATED    BY    GERMINATING   SEEDS. 

Fill  a  glass  funnel  of  medium  size  with  germinating  Peas 
or  blooming  heads  of  Leontodon,  Anthemis,  Bellis,  etc.;  into  which 
a  thermometer  graduated  to  \  degree  C.  has  been  thrust.  To  avoid 


46 


EXPERIMENTAL   PLANT  PHYSIOLOGY. 


loss  of  heat  as  far  as  possible,  cover  the  funnel  with  a  perforated 
glass  plate,  whereby  access  of  air  is  prevented.     The  carbon  dioxide 
FIG.  41.  formed  is  absorbed  by  a  solution  of 

potassium  hydrate  which  is  placed  in 
a  glass  dish  under  the  funnel.  As  a 
comparison  place  near  this  apparatus 
a  thermometer  in  the  free  air  which 
will  not  be  affected  by  the  heat  of 
the  plants.  To  obtain  the  most  uni- 
form temperature  for  both  thermom- 
eters cover  each  with  a  large  bell-jar. 

37.  Respiration,  Essential  to 
Growth  and  Dependent  on  Air.— The 
conversion  of  food  into  living  sub- 
stance  is   possible    by   means    of 
respiration  only.   The  higher  plants 
may  carry  on  a  certain  amount  of 
intramolecular  respiration  and  thus 
accomplish    a    small     amount    of 
growth.     For  the  normal  development  of  the  plant,  however, 
it  is  necessary  that  it  have  access  to  the  free  oxygen  of  the  air. 
EXPERIMENT  54. 

OXYGEN    NECESSARY    FOR    RESPIRATION. 

Fill  two  respiration-tubes  of  100  cc. 
capacity  with  water  which  has  been 
boiled  to  drive  off  the  dissolved  air.  In 
the  bulbs  of  each  insert  a  half-dozen 
seeds  of  Pea  or  Wheat,  and  invert  over 
a  dish  of  mercury.  Twenty-four  hours 
later  displace  nearly  all  of  the  water  in 
one  tube  with  hydrogen  and  the  other 
with  air.  The  seeds  in  hydrogen  do  not 
germinate,  while  those  in  air,  which  are 
able  to  obtain  their  customary  supply  of 
oxygen,  develop  normally.  To  obtain 
the  hydrogen,  place  a  few  grams  of  gran- 
ulated zinc  in  a  flask  or  bottle,  and  cover  to  a  depth  of  5  cm. 


(Sachs.) 


A,   filled    with    hydrogen, 
and 


RESPIRATION  AND    OTHER   FORMS   OF  METABOLISM.   47 

with  diluted  sulphuric  acid.  Close  the  mouth  of  the  flask  with  a 
cork  stopper  through  which  extends  a  short  section  of  glass  tubing. 
To  the  outer  end  of  this  attach  a  section  of  rubber  tubing  30  cm. 
in  length.-  The  free  end  of  the  rubber  tube  should  be  fitted  with 
a  small  piece  of  glass  tubing  drawn  to  a  point  and  bent  at  an  angle 
of  45  degrees  for  introducing  the  gas  into  the  respiration-tube. 
(Fig.  42.) 

38.  Fermentation.— Perennial  plants  which  grow  in  temper- 
ate climates  store  up  a  supply  of  reserve  food  in  the  roots, 
rhizomes,  or  stems,  to  serve  as  building  material  at  the  begin- 
ning of  the  next  vegetative  period.  The  seedling  cannot 
obtain  nourishment  from  the  soil  and  air  during  the  first  period 
of  its  development,  because  its  roots  are  not  sufficiently  de- 
veloped, and  because  it  has  not  yet  enough  chlorophyll  to  build 
up  food,  by  aid  of  the  sunlight.  Before  the  solid  reserve  sub- 
stances can  become  of  use  to  the  plant  they  must  be  dissolved, 
and  transported  by  diffusion  where  they  are  needed.  The 
solution  of  the  reserve  food  is  accomplished  by  means  of  fer- 
ments or  enzymes,  which  by  their  presence  induce  changes  in 
organic  compounds  {fermentation)  without  themselves  being 
thereby  in  any  way  affected.  On  account  of  this  last  property 
a  small  amount  of  enzyme  may  cause  fermentation  in  a  large 
quantity  of  -the  substance  acted  upon.  In  the  germination  of 
a  seed,  external  moisture  and  temperature  stimulate  the  proto- 
plasm to  form  an  enzyme  which  dissolves  the  solid  starch, 
protein,  or  fat  in  the  storage  cells.  The  solution  is  diffused 
into  the  growing  cells  of  the  young  plant  where  it  is  used  in 
building  up  protoplasm.  The  starch  formed  in  leaves  under- 
goes solution  and  transportation  in  a  similar  manner.  Diastase, 
the  enzyme  which  changes  starch  into  maltose,  is  perhaps  the 
most  widely  distributed  ferment. 


48  EXPERIMENTAL   PLANT  PHYSIOLOGY. 

EXPERIMENT    55. 

ACTION  OF  DIASTASE. 

Place  10  grams  of  seed  of  Barley  in  a  germinator  for  36  hours, 
or  until  the  radicles  are  .5  cm.  in  length.  Grind  fine,  in  an  ordi- 
nary coffee-mill,  and  add  to  three  parts  of  water.  After  a  time  filter 
and  mix  the  filtrate,  which  now  contains  diastase,  with  a  fifth  part 
of  very  thin  starch  paste  (i  gram  starch,  TOO  grams  water).  A 
sample  of  this  mixture  is  colored  blue  on  treatment  with  iodine,  a 
sample  taken  later,  violet,  then  brown,  and  finally  one  taken  after 
two  or  three  hours  is  colorless,  demonstrating  that  all  the  starch 
has  been  transformed  into  maltose  or  sugar  by  the  diastase  present 
in  the  germinated  seed. 

Cut  thin  sections  of  the  seeds  at  the  beginning  of  the  experiment 
and  determine  the  appearance  and  characteristics  of  the  starch- 
grains.  Make  a  similar  examination  24  and  48  hours  later.  Allow 
germination  to  proceed  in  a  few  seeds,  and  examine  4  days  later. 
The  starch-grains  are  gradually  corroded  and  dissolved  by  the 
diastase  formed. 

EXPERIMENT  56. 

TRANSLOCATION   OF    STARCH. 

A  Tropseolum  plant  whose  leaves  are  rich  in  starch  is  placed  in 
the  dark  after  some  of  its  leaves  have  been  cut  off.  The  excised 
leaves  are  likewise  placed  in  the  dark  in  a  moist  room  or  under  a 
bell-jar.  After  a  few  days  test  some  of  the  excised  leaves  and  those 
remaining  on  the  plant  for  starch.  Those  on  the  plant  show  some 
starch,  mostly  in  the  nerves,  while  those  excised  show  starch  in  the 
other  parts  as  well,  because  they  could  not  transfer  it  to  other 
organs. 

EXPERIMENT  57. 

FORMATION  AND   TRANSLOCATION   OF   STARCH. 

On  a  well-developed  plant  of  Tropaeolum  majus  standing  in  the 
sunlight  in  the  forenoon,  darken  portions  on  some  healthy  leaves,  by 
means  of  cork  plates,  fastened  on  opposite  sides  by  pins.  On  the 
afternoon  of  the  following  day  cut  off  the  leaves  and  boil  in  water 
in  a  porcelain  dish  for  a  few  minutes,  to  kill  the  protoplasm.  Ex- 
tract the  coloring  matter  by  alcohol  many  times  renewed.  The  de- 
colorized leaves  are  now  saturated  with  alcoholic  iodine  in  a  porce- 


RESPIRATION  AND    OTHER  FORMS   OF  METABOLISM.    49 

lain  dish,  whereupon  they  will  be  colored  a  deep  blue  except  in 
shaded  portions.  Since,  substantially,  starch-formation  in  the  leaf 
proceeds  by  day  only,  and  the  solution  and  translocation  at  night 
as  well,  the  places  exposed  to  the  sunlight  contain  enough  starch  to 

FIG.  43.  b 


<t,  Tropaeolum  leaf  to  which  are  attached  two  pieces  of  cork  to  prevent  pho- 
tosynthesis. (Detmer.)  b,  same  after  removal  of  cork,  treated  with 
iodine. 

give  the  microscopic  reaction  with  iodine.  It  was  taken  away  from 
the  shaded  places  at  night,  however,  and  could  not  be  replaced. 
(Fig.  43-) 

39.  Changes  in  Color. — Many  changes  in  the  chemical  com- 
position of  substances  in  the  plant  are  accompanied  by  corre- 
sponding changes  in  color.  (See  Chlorophyll,  Par.  30.)  Flow- 
ers which  are  blue  when  fully  opened  were  originally  red  in  the 
bud.  The  sap  was  acid  at  first  and  became  alkaline  as  the  result 
of  metabolic  changes.  Leaf-colors  offer  similar  conditions, 
although  the  changes  in  color  here  are  sometimes  due  to  the 
oxidation  of  chlorophyll  and  other  coloring  matter  in  the  cells. 
EXPERIMENT  58. 

RELATION  OF  RED  AND  BLUE  COLORS  OF  FLOWERS. 

Immerse  a  leaf  of  Begonia  bearing  red  hairs,  for  a  short  time  in 
a  weak  solution  of  ammonia.  The  hairs  become  blue.  Place  a 
blue  petal  of  Myosotis  in  a  i$  solution  of  acetic  acid.  It  becomes 
reddish.  Express  the  sap  from  a  handful  of  petals  of  Roses  or 
Peonies.  Collect  in  a  test-tube  and  add  a  few  drops  of  ammonia. 
A  blue  color  results.  Add  some  acid.  The  red  color  is  restored. 
Observe  the  different  colors  assumed  by  leaves  in  the  autumn. 


CHAPTER  V. 

IRRITABILITY. 

40.  Nature  of  Irritability. — The  term  irritability  designates 
that  property  of  plants  by  which  they  respond  to  certain  influ- 
ences known  as  stimuli.     The  stimuli  may  be  either  internal 
forces  set  in  operation  by  metabolic  activity  or  external  influ- 
ences, such  as  gravity,  light,  temperature,  electricity,  moisture, 
and  mechanical  contact.     The  plant  may  react  in  two  ways, 
first,  by  changes  in  the  structure,  form,  and  size  of  its  organs  ; 
second,  by  motion  or  change  in  position  of  its  organs  or  of  the 
protoplasmic  bodies  in  its  cells.    The  reactions  of  the  first  class 
concern  growth ;   those  of  the  second  class  result  in  placing 
plant  or  cell  organs  in  certain  positions  relative  to  the  direction 
of  the  stimulus.     Thus  a  plant  grown  in  darkness  develops  its 
stems  and  leaves  in  quite  different  form  and  structure  from 
one  grown  in  the  open  air  (Fig.  67).     Light,  then,  affects  the 
structure  and  form  of  plants  by  what  is  known  as  its  formative 
or  tonic  influence.    (See  Chapter  VI.)    Light  also  causes  shoots 
to  bend  toward  its  source  in  such  manner  as  to  place  their 
axes  parallel  to  the  light-rays.     This  is  termed  its  directive  in- 
fluence.    A  stimulus  may  give  rise  to  reactions  of  both  kinds 
at  the  same  time,  and  they  cannot  always  be  easily  and  dis- 
tinctly separated  by  experiments. 

41.  Perceptive  and  Motor  Zones. — The  action  of  an  external 
stimulus  on  one  part  of  a  plant  does  not  necessarily  cause  a 
movement  in  that  part,  but  the  impulse  may  be  transmitted  to 
a  region  more  or  less  distant.    Thus,  for  instance,  a  touch  on  the 

leaf-blade  of  a  Mimosa  (Sensitive-plant)  causes  no  contraction 

50 


IRRITABILITY. 


in  the  leaf-blade,  but  the  impression  is  transmitted  to  the  pul- 
vinus  at  the  base  of  the  leaf  or  leaflet  and  produces  movement. 
The  region  which  receives  the  stimulus  is  designated  the/^r- 
ceptive  zone,  and  the  one  which  causes  the  movement,  the 
motor  zone.  The  two  may  coincide  in  position. 

42.  Geotropism. — The  power  by  which  a  plant  responds  to 
the  influence  of  gravity  is  termed  geotropism.  The  response 
of  an  organ  to  this  stimulus  may  occur  in  three  ways,  as  follows. 
(i)  The  organ  may  point  its  apex  toward  the  centre  of  the  earth, 
the  source  of  gravity,  in  which  instance  it  is  said  to  be  pro- 
geotropic.  This  action  is  generally  manifested  by  primary 
roots.  (2)  It  may  point  its  axis  away  from  the  source  of  gravity, 
directly  upward,  when  it  is  said  to  be  apogeotropic.  Erect 
shoots  are  generally  apogeotropic.  In  general  organs  of 
radial  structure  exhibit  one  of  these  two  forms  of  geotropism. 
(3)  The  organ  may  place  its  axis  in  a  horizontal  position,  at 
right  angles  to  the  force  of  gravity,  when  it  is  said  to  be  dia-* 
geotropic.  This  is  characteristic  of  the  larger  number  of  bilateral 
organs,  such  as  leaves,  although  also  shown  by  organs  of'  radial 
structure,  such  as  branches  of  stems,  secondary  roots,  etc. 

». 
EXPERIMENT  59. 

PROGEOTROPISM. 

To  a  cork  in  the  top  of  a  bell-glass  fasten  a  seedling  of  a 
Bean  with  the  radicle  which  is  i 

to  2  cm.  in  length  in  a  horizontal  FIG.  44- 

position.  In  a  few  hours  the  tip 
is  found  to  be  pointing  downward 
more  or  less  directly.  (Fig.  44.) 

EXPERIMENT  60. 

GEOTROPIC    REACTIONS    OF    SEEDLINGS. 

Place  a  layer  of  sawdust  be- 
tween two  horizontal  parallel  rings     Progeotropism  of  seedling.     (Det- 

mer.)      b,   dish  partly  filled  with 
covered  with  wide-meshed  gauze.       water ;  g,  bell-glass  ;  s,  seedling. 


$2  EXPERIMENTAL   PLANT  PHYSIOLOGY. 

Plant  seeds  of  the  Pea,  Bean,  or  Corn  in  the  sawdust.     The  roots 
FIG.  45.  pass  through  the  meshes  downward   and  the 

shoots  upward.  Invert  the  apparatus  and  these 
organs  will  bend  in  the  opposite  directions. 
(Fig.  450 

FIG.  46. 


Geotropism  of  roots  and  Apogeotropism   of   leaves   of   Onion, 

shoots.     (After  Oels.)  (Frank.) 

EXPERIMENT  61. 

APOGEOTROPISM. 

Place  a  Tulip,  Hyacinth,  Onion,  or  Fritillaria,  which  is  growing 
rapidly,  in  a  horizontal  position.  In  a  short  time  the  leaves  curve 
directly  upward.  (Fig.  46.) 

EXPERIMENT  62. 

DIAGEOTROPISM. 

Observe  the  opening  flower-buds  of  a  Narcissus,  which  at  first  are 
erect,  but  later  the  perianth-tube  assumes  a  horizontal  position. 
After  they  have  attained  this  position  lay  the  pot  on  its  side  with 
the  leaves  and  stems  horizontal  and  the  perianth-tube  pointing 
downward.  In  10  hours  the  pedicels  will  have  again  curved  to 
place  the  perianth-tube  in  the  same  position  as  before. 

43.  Perceptive  and  Motor  Zones  of  Roots. — The  stimulus  of 
gravity  is  received  by  a  sensitive  portion  near  the  tip  of  a  root 
(the  perceptive  zone),  and  an  impulse  is  conveyed  to  a  region 
several  millimeters  distant  which  curves  (the  motor  zone). 


IRRITABILITY. 


The  motor  zone   in  this  instance  is  located  in  the  region  of 
most  active  growth.    (See  Experiment  86.) 
EXPERIMENT  63. 

PERCEPTIVE   ZONE   OF    ROOTS. 

Repeat  Experiment  59  after  cutting  away  a  portion  of  the  tip 
of  the  root  i  to  2  mm.  in  length.  The  root  does  not  now  respond 
to  gravity,  and  shows  no  movement  until  the  tip  is  rehabilitated,  when 
it  curves  downward  in  a  natural  manner. 

EXPERIMENT  64. 

MOTOR   ZONE   OF    ROOTS. 

With  a  fine  brush  carefully  mark  off  equal  spaces  (2  to  3  mm.)  on 
FIG.  47.  the  primary  root  of  a  seed- 

ling of  Phaseolus  (Bean). 
•~  ~  ~T  iMJm?Vrs-  Suspend  in  a  horizontal 
position  in  a  moist  chamber. 
In  a  day  note  the  region 
in  which  curvature  has 
occurred,  and  its  distance 
from  the  tip. 

EXPERIMENT  65. 

MOTOR   ZONES    OF   CULM    OF 
GRASS. 

Cut  a  length  of  12  cm. 
from  a  vigorously  growing 
culm  of  Grass.  Place  in  a 


Motor  zone  in  roots.     (Pfeffer.) 


horizontal  position  in  a  moist  chamber  with  one  end  imbedded 
in  sand.  Six  hours  later  note  the  region  of  curvature.  The  motor 
zone  will  be  found  in  the  FIG.  48. 

pulvinus  like  internodes.  (Fig. 
48.) 

EXPERIMENT  66. 

FORCE     OF     CURVATURE. 

Fasten  a  seedling  of  Pea 
with  a  radicle  i  cm.  in  length 
to  a  piece  of  cork  attached  to 
the  side  of  a  vessel  containing 
mercury.  Place  the  seedling  in  such  position  that  the  root  is 


Curvature  of  Culm  of  Grass. 
(After  Oels.) 


54 


EXPERIMENTAL   PLANT  PHYSIOLOGY, 


horizontal  and  the  tip  is  in  contact  with  the  mercury.  Pour  in 
enough  water  to  form  a  thin  layer  on  top  of  the  mercury.  The 
root  will  bend  downward  with  such  FIG.  49. 

force  as  to  penetrate  the  mercury. 
(Fig.  49.) 

44.  Influence  of  Gravity.  — 
Gravity  acts  in  a  vertical  direc- 
tion and  with  a  force  directly 
proportioned  to  the  mass  of  the 
body  acted  upon.  In  plants  the  amount  and  rapidity  of  the 
curvature  of  an  organ,  in  response  to  gravity,  depends  on 
its  stage  of  development  and  the  angle  which  its  axis  forms 
with  the  vertical.  Progeotropic  organs  respond  most  rapidly 

FIG.  50. 


Root  of  seedling  penetrating 
mercury.     (Sachs.) 


Seedlings  on  a  revorving  wheel  driven  by  a  clock.     (After  Oels.) 


IRRITA  BILIT  Y.  55 

when  their  tips  are  pointing  upward  at  an  angle  of  45  degrees 
from  the  vertical ;  apogeotropic  organs  respond  most  readily 
when  pointing  downward  at  an  angle  of  45  degrees ;  and  dia- 
geotropic  organs  respond  with  equal  facility  in  either  position. 
The  action  of  gravity  upon  a  plant  may  be  neutralized  by  plac- 
ing it  on  the  periphery  of  a  wheel  revolving  in  a  vertical  plane, 
or  by  turning  the  plant  on  its  own  axis  in  a  horizontal  positions 

EXPERIMENT  67. 

NEUTRALIZATION    OF    INFLUENCE   OF    GRAVITY. 

Take  the  hands  from  a  large  wall-clock  whose  dial  is  parallel  to 
the  rays  of  sunlight  (to  avoid  the  disturbing  action  of  light)  and 
fasten  to  the  prolonged  axis  a  cork  plate  10  cm.  in  diameter,  or  a 
wheel  to  the  periphery  of  which  are  attached  pieces  of  cork. 
Fasten  seedlings  of  Pea  to  the  cork  in  various  positions  by  means 
of  pins,  and  set  the  clock  in  motion.  Twenty-four  hours  later  each 
seedling  will  be  found  to  be  growing  in  the  position  in  which  it  was 
placed,  and  no  marked  curvatures  in  any  of  the  organs  can  be 
noticed.  (Fig.  50.) 

Remark. — The  revolving  wheel  must  be  partially  immersed  in  water  or 
placed  in  a  spray  to  keep  the  seedlings  moist.  A  small  American  clock 
can  be  used  instead  of  the  large  clock  shown  in  the  figure. 

45.  Replacement  of  Gravity. — When  gravity  is  overcome  by 
another  force,  the  seedling  tends  to  place  its  axis  parallel  to  this 
new  force  in  the  same  manner  as  toward  gravity  in  its  normal 
position.  The  force  which  acts  upon  a  plant  fastened  to  a 
rapidly-revolving  wheel  acts  in  a  tangential  direction  ;  conse- 
quently the  plant  tends  to  place  its  axis  parallel  to  the  tangent. 
This  is  true  of  plants  rotated  in  a  vertical  plane  at  a  speed  of 
100  to  300  revolutions  per  minute  with  a  wheel  having  a  radius 
of  6  to  20  cm.  If  a  plant  is  rotated  in  a  horizontal  plane  at 
this  speed,  centrifugal  force  tends  to  cause  the  shoot  axis  to 
lie  in  a  horizontal  plane,  while  gravity  tends  to  cause  the 
axis  to  take  a  vertical  position.  In  this  instance  the  axis  will 
take  a  position  between  the  direction  of  these  two  forces. 


50  EXPERIMENTAL   PLANT  PHYSIOLOGY. 

The  roots  will  point  outward  and  downward,  and  the  shoots 
upward  and  inward.  These  positions  will  be  taken  by  rapidly- 
growing  seedlings  in  5  or  6  hours. 

EXPERIMENT  68. 

REPLACEMENT  OF  GRAVITY  BY  CENTRIFUGAL  FORCE. 

If  a  water  system  is  at  hand,  the  rapid  rotation  of  the  wheel  or  disk 
holding  seedlings  may  be  accomplished  by  the  following  apparatus 

FIG.  51. 


Centrifugal  apparatus.  (After  Oels.)  A,  heavy  board  base  ;  B,  cork  hold- 
ing the  sealed  end  of  a  glass  tube  which  serves  as  a  bearing  for  the  end 
of  the  axis  of  the  wheel. 

and  the  seedlings  may  be  kept  moist  during  the  experiment.  Upon  a 
wire,  40  centimeters  long,  the  size  of  a  knitting-needle,  are  strung  at 
equal  distances  a  number  of  circular  cork  plates,  3  cm.  in  diameter 
and  a  half-centimeter  thick.  The  wire  is  now  bent  in  the  form  of  a 


IRRITA  BILIT  Y.  $7 

circle,  the  ends  brought  together  through  a  piece  of  cork  and  united. 
Four  brass  wires  serve  as  spokes,  while  the  hub  is  made  from  a 
heavy  cork.  The  axis  is  also  made  from  a  piece  of  wire,  about  15 
cm.  in  length,  in  order  that  the  seedlings  may  have  enough  space 
for  their  curvatures.  The  axis  rests  in  bearings  made  of  the  ends 
of  sealed  glass  tubes  (Fig.  51,^),  which  are  fastened  in  stationary 
corks  by  means  of  sealing  wax.  If  now  a  sufficient  stream  is  al- 
lowed to  fall  perpendicularly  on  the  cork  plates  on  one  side,  a 
rapidity  of  revolution  will  be  secured  that  will  in  five  or  six  hours 
effect  a  noticeable  change  in  position  of  the  seedlings,  which  have 
been  placed  with  their  roots  toward  the  center.  If  large  casks  are 
at  hand,  the  experiment  may  be  carried  on  without  a  water  system. 
A  glass  tube  of  an  internal  diameter  of  4  mm.  with  a  fall  of  i  meter 
of  the  water  will  furnish  a  stream  that  will  revolve  the  wheel  more 
than  twice  per  second,  which  is  entirely  sufficient  for  the  experi- 
ment. The  replacement  of  the  water  is  necessary  for  the  continua- 
tion of  the  experiment. 

46.  Heliotropism,  Thermotropism,  etc. — Radiant  energy  in 
the  form  of  heat,  light,  and  electricity  exercises  a  very  marked 
directive  influence  on  the  position  of  plant-organs.  The  effect 
of  sunlight  is  much  better  known  than  that  of  the  other  stimuli 
acting  on  the  plant.  It  is  a  matter  of  general  observation 
that  the  shoots  of  a  large  number  of  plants  bend  toward  the 
light.  A  close  inspection  shows  that  various  organs  respond 
in  a  different  manner  to  light,  as  also  to  gravity.  Thus 
many  roots  direct  themselves  away  from  the  source  of  light 
(apheliotropism],  trailing  shoots,  leaves,  etc.,  at  right  angles  to 
the  rays  (diaheliotropism),  while,  as  noted  above,  others,  such 
as  stems,  bend  toward  the  light  (proheliotropism).  It  will  be 
seen  that  while  the  force  of  gravity  acts  always  in  thfe  same 
direction,  the  line  of  light-rays  from  the  sun  moves  through 
an  angle  of  180  degrees  daily.  This  change  of  the  position  of 
the  source  of  light  causes  corresponding  movements  in  helio- 
tropic  organs.  Sunlight  affords  two  separate  stimuli  to  the 
plant :  one  from  the  blue-violet  rays  which  causes  helio- 


So  EXPERIMENTAL   PLANT  PHYSIOLOGY. 

tropic  movements,  and  one  from  the  red  end  of  the  spectrum 
which  causes  heat  or  thermotropic  movements,  as  may  be 
shown  by  Experiment  72.  The  purpose  of  the  heliotropic  as 
well  as  all  other  movements  of  plants  is  doubtless  that  of  plac- 

FIG.  52. 


\ 


Diagram  of  light  positions  of  leaves.     The  arrows  denote  the  direction  of 
the  rays.     (Vochting.) 

ing  the  plant-organ  in  the  position  best  suited  to  the  perform- 
ance of  its  functions.  Heliotropic  movements  place  the  leaves 
in  a  position  most  favorable  for  photosynthesis  and  transpira- 
tion. 


IRRITABILITY. 


59 


EXPERIMENT  69. 

PROHELIOTROPISM. 

Place  a  Malva  or  Helianthus  grown  in  a  pot  in  the  open  air, 
near  a  window  with  a  southern  exposure.  The  leaves  gradually 
assume  the  definite  positions  shown  in  Fig.  52. 

EXPERIMENT  70. 

HELIOTROPIC   MOVEMENTS    OF    ROOTS   AND   SHOOTS. 

Fasten  seedlings  of  Sinapis  alba  (Mustard)  or  Phaseolus  multiflo- 
rus  (Bean)  on  a  piece  of  tulle  stretched  lightly  across  a  glass  vessel 

FIG.  53- 


Dark  chamber  with  a  tube  opening  in  one  end.     (Schleichert.) 
filled  with  spring-water.     After  the  roots  and  stems  have  attained  a 


FIG.  54- 


Seedling  of  Mustard  grown  under 
one-sided     illumination, 
mer.) 


length  of  i  cm.  place  the  apparatus 
under  a  pasteboard  box  lined  with 
black  paper,  through  which  the 
light  may  gain  entrance  by  a  small 
aperture.  In  a  few  hours  the  roots 
and  stems  will  be  influenced  as  de- 
scribed above.  (Figs.  53  and  54.) 
'  EXPERIMENT  71. 

HELIOTROPIC  MOVEMENTS    OF    LEAVES. 

Bend  and  fasten  in  a  horizontal 
position  an  upright  well-leaved 
branch  of  the  Maple,  or  a  whole 
Helianthus  plant,  in  the  open  air. 
(Det-  Soon  the  leaves  which  were  previ- 
ously horizontal,  and  perpendicu- 


lar to  the  shoot  on  all  sides,  show  peculiar  torsions  of  the  petioles, 


60  EXPERIMENTAL   PLANT  PHYSIOLOGY. 

which,  so  far  as  they  are  capable  of  growth,  finally  result  in  the 
placing  of  the  leaves  in  a  horizontal  position,  but  parallel  to  the 
shoot  and  one  another.  (Fig.  55.) 

EXPERIMENT  72. 

EFFECT    OF    RED    AND    BLUE    LIGHT. 

Of  two  equally  sensitive  seedlings,  place  one  in  a  chamber  (Fig. 
55),  with  one  side  of  yellow  glass,  and  the  other  in  a  similar  chamber, 
with  one  side  of  blue  glass.  The  heliotropic  movement  is  much  more 
marked  in  the  blue  light.  Instead  of  the  colored  plates,  use  glass 
vessels  with  parallel  walls,  filled,  one  with  a  solution  of  bichromate 

FIG.  55- 


Shoot  of  Sunflower  which  has  been  in  a  horizontal  position  several  days. 

(After  Oels.) 

of  potassium,  the  other  with  a  solution  of  ammonia-copper-oxide. 
Only  small  plants  can  be  used  in  the  experiment. 

EXPERIMENT  73. 

HELIOTROPIC    REACTION    OF    PLANT    WITH    GRAVITY    NEUTRALIZED. 

If  the  influence  of  gravity  is  removed  from  a  plant  as  in  Experi- 
ment 67,  and  the  light  allowed  to  fall  parallel  to  the  axis  of  the 
plant,  the  roots  and  shoots  will  be  seen  to  take  opposite  direc- 
tions in  a  plane  parallel  to  its  rays. 

EXPERIMENT  74. 

THERMOTROPISM. 

Grow  seedlings  of  Corn  in  a  pot,  and  place  in  a  position  where 
the  light  will  be  received  perpendicularly.  At  a  distance  of  40  cm. 
place  a  sheet  of  smoked  tin  which  is  kept  warm  by  a  spirit-lamp. 
In  twenty-four  hours  the  shoots  will  have  inclined  toward  the 
source  of  the  heat.  Repeat  the  experiment  with  Peas. 


IRRITA  BILIT  Y.  6 1 

47.  Periodic  Movements. — The  sun  is  continually  changing 
its  position  during  the  day  ;  consequently  if  a  leaf  remains  in  a 

FIG.  56. 


Day  and  night  positions  of  leaflets  of  Bean.     (Detmer.) 
fixed  position  it  receives  the  maximum  heat  and  light  at  one 
moment  only.     It  is  found  that  leaves  not  only  exhibit  move- 
ments corresponding  to  the  heat  and  light  received,  but  also 
FIG.  57.  assumed  certain  positions 

to  avoid  excess  or  loss  of 
heat.  An  organ  loses  or 
receives  the  least  heat 
when  its  long  axis  is 
vertical.  A  great  num- 
ber of  these  movements 
have  been  described  as 
"  sleep  movements." 
EXPERIMENT  75. 

SLEEP   MOVEMENTS. 

Observe  the  positions  of 

Sleep  position  of  leaves  of  Oxalis  induced     the  leaflets  of  a  seedling  of 
by  artificial  darkness.     (Hansen.)  Bean  Qr  of  an    Qxalis>  grQW- 

ing  in  the  sunlight,  at  8  A.M.,  i  P.M.,  and  6  P.M.  Determine  whether 
these  positions  are  due  to  light  or  heat  by  use  of  the  dark  chamber. 
(Figs.  56  and  57.) 


62  EXPERIMENTAL   PLANT  PHYSIOLOGY. 

48.  Hydrotropism.—  The    moisture   of    the  medium  which 
surrounds   the   plant   induces   movements   in    certain   organs 
either  toward  or  away  from  the  source  of  the  moisture.     The 
property  of  an  organ  by  which  it  reacts  to  moisture  is  termed 
hydrotropism.     By  this  power  roots  direct  their  apices  toward 
portions  of  the  soil  containing  the  proportion  of  moisture  best 
suited  to  their  specific  needs. 

EXPERIMENT  76. 

HYDROTROPISM    OF     ROOTS. 

Cover  a  zinc  box,  5  cm.  wide  and  20  cm.  long,  open  on  two  sides, 
with  gauze  after  it  has  been  filled  with  moist  sawdust,  containing 
swollen  seeds  of  Bean,  Pea,  or  FIG.  58. 

Corn.  Suspend  the  apparatus 
under  a  pasteboard  box,  so  that  it 
hangs  at  an  angle  of  45  degrees. 
After  a  time  the  roots  issue  through 
the  openings  in  the  gauze  beneath  ; 
they  do  not  follow  geotropism  and 
grow  directly  downward,  however, 
but  press  against  the  layer  of  moist 
sawdust.  Place  the  apparatus  in  a 
damp  chamber  where  the  moisture 
is  equal  in  all  directions  from  the 
roots,  and  they  grow  directly  down- 
ward in  response  to  the  stimu- 
lus of  gravity.  In  this  case  the 
roots  receive  the  same  stimulus 
from  moisture  in  all  directions, 
and  in  consequence  no  reaction  to 
it  is  shown.  They  are  free  to  re-  Hydrotropism  of  roots.  (Detmer.) 
spond  to  their  progeotropic  tendency. 

49.  Contact  Movements. — Many   plants   will    exhibit    move- 
ments so  rapid  as  to  be  visible  to  the  naked  eye  when  touched 
or  struck  with  any  hard  object.     These  movements  serve  vari- 
ous purposes  in  different  groups  of  plants.     In  some  instances, 
as  in  the  Mimosa,  this  is  a  device  for  protecting  the  leaves 


IRRITABILITY.  63 

from  injury.  By  this  "  sensitiveness  "  of  tendrils,  climbing 
plants  are  able  to  attach  themselves  to  supports  and  lift  their 
leaves  to  sunlight.  In  certain  carnivorous  plants,  such  as  Dro- 
sera  and  Dionaea,  the  rapid  movement  of  the  tentacles  and 
leaves  enables  these  plants  to  capture  insects  which  are  held 
and  whose  substance  is  absorbed  by  the  plant. 

EXPERIMENT  77. 

MOVEMENTS    OF   SENSITIVE-PLANTS. 

Grow   Mimosa   pudica   (Sensitive-plant)   from   seed,  in   a  pot. 
Moisture  and  temperature  of  about  20°  C.  are  necessary  for  the  welfare 

FIG.  59- 


Mimosa  pudica.     The  leaf  on  the  left  is  in  a  normal  position  ;  the  one  on  the 
right  has  been  stimulated.     (Detmer.) 

of  the  plant  ;  consequently  it  should  be  kept  under  a  bell-jar  slightly 
raised  at  one  side  to  allow  for  ventilation,  and  placed  in  the  sun- 
shine. Try  the  following  experiments  :  a.  Jar  the  entire  plant  by 
striking  the  pot.  In  a  few  seconds  the  leaves  take  the  position 
shown  in  Fig.  59.  b.  Strike  one  of  the  terminal  leaflets.  The  pairs 
of  leaflets  fold  up  together  in  succession,  and  finally  the  whole  leaf 
sinks  on  its  petiole,  c.  Touch  the  upper  side  of  the  pulvinus  with 
a  pointed  object.  No  movement  follows,  d.  Touch  the  under  side 
in  like  manner.  A  movement  results,  e.  With  a  sharp  knife  cut  off 
the  petiole  just  above  the  pulvinus.  A  drop  of  water  issues  from  the 
lower  surface,  which  in  an  uninjured  leaf  would  pass  into  the  leaf-stalk. 


^4  EXPERIMENTAL   PLANT  PHYSIOLOGY. 

EXPERIMENT  78. 

MOVEMENTS   OF    STAMENS. 

Touch  stamen-filaments  of  Centaurea,  Carduus,  or  Cichorium. 
The  filaments  contract. 

FIG.  60. 


Tendrils  of  Bryony.    (Kerner.)    a,  young  tendrils  ;  b  b,  nearly  mature  and 
very  highly  irritable  ;  c  c,  two  tendrils  which  have  intertwined. 

EXPERIMENT  79, 

CURVATURE    OF   TENDRILS. 

Touch  a  tendril  of  the  Passion-flower,  Bryony,  or  Squash  on 
the  concave  surface  near  the  tip  with  a  pencil  and  observe  carefully. 


IRRITABILITY. 


In  a  time  varying  from  30  seconds  to  several  minutes  a  curvature  is 
begun.  Note  rapidity,  extent,  and  duration.  Place  a  small  rod  in 
contact  with  the  tendril.  In  a  few  hours  it  will  have  coiled  around 
it.  Observe  the  formation  of  spirals  in  the  free  portion  of  the 
tendril.  (Fig.  60.) 

EXPERIMENT  80. 

RELATION  OF  HARDNESS  OF   OBJECTS  TO  CURVATURE    PRODUCED    IN   TENDRILS. 

Test  the  effect  of  water,  mercury,  soft  gelatine,  glass,  iron,  and 
wooden  objects  when  brought  in  contact  with  tendrils. 

EXPERIMENT  81. 

ACTION    OF   LEAVES   AND   TENTACLES    OF    CARNIVOROUS    PLANTS. 

Obtain  several  plants  of  Sundew  (Drosera)  from  the  swamps. 
In  digging  them,  care  should  be  taken  to  leave  a  large  mass  of  the 
soil  on  the  roots  of  each  so  that  their  growth  may  not  be  greatly 
disturbed.  Cover  with  a  bell-jar  and  place  in  the  sunlight.  Touch 


FIG.  61. 


FIG.  62. 


Leaf  of  Dionaea  expanded. 
(Kerner.) 


Leaf  of  Drosera  with  right  leaf  halt 
contracted.     (Darwin.) 


the  tentacles  with  small  pieces  of  a  large  variety  of  substances, 
wood,  sugar,  starch,  paste,  alkali,  meat,  bits  of  stone,  etc.,  and  note 
to  what  substances  the  tentacles  react  and  the  rapidity  of  move- 
ment. Repeat  with  Dionsea. 


66  EXPERIMENTAL   PLAN 'T  PHYSIOLOGY. 

50.  Circumnutation. — If  the  tip  of  a  shoot  of  some  rapidly, 
growing  plant,  such  as  the  Pea  or  Bean,  is  kept  under  observa- 
tion for  several  minutes,  it  will  be  seen  that  it  slowly  changes 
position ;  and  if  the  time  of  observation  is  extended,  it  will  be 
found  that  it  inclines  successively  toward  every  point  in  the 
horizon.     In  some  plants  the  movement  is  in  the  same  direc- 
tion as  the  hands  of  a  watch,  and  in  others  it  is  in  the  contrary 
direction.     This  nutatory  movement  of  growing  tips  is  quite 
generally  distributed  among  plants,  but  it  is  most  marked  in 
twining  stems.     The  causes  which  produce  the  movement  are 
chiefly  inequalities  in  growth  extension  of  the  sides  of  the 
stem,  and  the  reaction  to  the  influence  of  gravity. 

EXPERIMENT  82. 

CIRCUMNUTATION    OF    SHOOTS   AND   TENDRILS. 

Note  the  positions  of  a  growing  tendril  of  the  Gourd,  Pea, 
Bryony,  or  Wild  Balsam  Apple  at  intervals  for  three  hours. 

Plant  three  or  four  seedlings  of  the  Scarlet  Runner  or  commoi. 
Bean  at  equal  distances  from  one  another  in  a  circle  around  an 
upright  post.  Mark  the  successive  positions  of  the  tips  of  each 
until  it  becomes  twined  around  the  support. 

51.  Hygroscopic  Movements. — Many  plants  are  provided  with 
cells  which  take  up  or  lose  water  in  such  manner  as  to  give 
rise  to  very  marked  movements  in  the  organs  of  which  they 
form  a  part.     Such  cells  are  found  in  the  leaf-blades  of  a  large 
number  of  Grasses,  and  other  plants  which  inhabit  arid  regions. 
In  such  plants  this  is  a  provision  for  rolling  up  the  leaves  in 
a  form  which  will  prevent  undue  loss  of  moisture  from  the 
organ.     By  a  similar  action  many  anthers  open  and  allow  the 
escape  of  the  pollen,  and  fruit-capsules  allow  seeds  to  escape. 
In  the  latter  instance  sufficient  force  is  sometimes  furnished 
to  throw  the  seeds  to  a  distance  or  bury  them  in  the  soil. 


IRRITABILITY.  07 

EXPERIMENT  83. 

WARPING  OF  WOOD. 

Wipe  the  adhering  moisture  from  a  thin  piece  of  wood,  such  as 
a  cigar-box  lid,  which  has  lain  in  water  24  hours,  and  fasten  to 
another  piece  of  similar  size  which  is  air-dry,  by  means  of  a  number 
of  small  nails.  Lay  in  a  dry  place.  The  loss  of  moisture  from  the 
saturated  piece  will  cause  the  double  board  to  become  curved. 

Note  the  "  warping  "  of  unseasoned  timbers. 

EXPERIMENT  84. 

MOVEMENTS    OF    FRUIT-CAPSULE   OF    IMPATIENS. 

Bring  some  fruit  of  Balsamina  (Impatiens  noli  tangere)  which 
is  nearly  ripe  into  warm  dry  air.  (Hold 
at  a  distance  above  a  gas-flame.)  The 
outer  covering  of  the  fruit  contracts  and 
forcibly  ejects  the  fruit.  Make  sections 
of  the  portions  of  the  capsule,  and  describe 
the  action  of  the  hygroscopic  cells. 

EXPERIMENT  85. 

TWISTING   MOVEMENTS    OF   THE   BEAK   OF   AN 
ERODIUM    SEED. 

Erodium  seed  with  the  long 
beak  twisted  by  drying.        A   moist   seed   of   Erodium   is   placed 

with  the  point  in  a  damp  soil.     In  drying 

the  beak  curves  and  twists  in  a  spiral  form.  If  the  twisting  of  the 
beak  is  hindered  by  a  piece  of  wood  thrust  into  the  sand  beside  it,, 
the  force  will  be  exerted  upon  the  seed,  and  will  be  thrust  into  the 
sand  still  deeper.  (Fig.  63.) 


CHAPTER  VI. 

GROWTH. 

52.  Nature  of  Growth. — The  increase  of  the  living  substance 
of  an  organism  is  designated  growth,  and  it  is  generally  accom- 
panied by  an  increase  in  weight  and  size.  This  increase  does 
not,  however,  always  accompany  growth;  indeed,  it  was  de- 
monstrated in  Experiment  29  that  a  plant  may  grow  while 
losing  in  weight.  If  the  plant  is  accumulating  storage  material 
it  will,  on  the  other  hand,  undergo  an  increase  in  weight  not  in 
any  manner  connected  with  growth.  It  is  also  to  be  noted 
that  independent  changes  in  form  and  size  occur  which  are  due 
simply  to  alterations  in  the  force  of  turgor  and  in  the  extensi- 
bility of  the  cell-walls.  Lastly,  growth  does  not  consist  in  the 
formation  of  new  cells ;  on  the  contrary,  the  formation  of  new 
cells  is  a  result  of  growth. 

EXPERIMENT  86. 

MEASUREMENT    OF    GROWTH    EXTENSION. 

To  determine  the  increase  in  length  of  a  plant  the  simple  auxa- 
nometer  shown  in  Fig.  64  will  be  found  fairly  accurate.  This  appa- 
ratus consists  of  an  upright  stand  50  cm.  in  height  to  which  is 
attached  a  horizontal  arm  10  cm.  in  length.  To  the  end  of  this 
arm  is  attached  a  wooden  pulley  4  cm.  in  diameter,  in  such  manner 
that  it  will  turn  freely.  To  one  side  of  this  pulley  is  fixed  a  thin 
wooden  pointer  20  cm.  in  length.  This  pointer  is  made  from  a 
strip  not  more  than  2  mm.  in  thickness,  and  has  wrapped  around 
the  larger  end  at  g  a  sufficient  quantity  of  tinfoil  to  balance  the 
longer  end.  A  curved  paper-scale  ruled  to  2  mm.  is  held  by 
another  stand  near  the  tip  of  the  pointer.  A  linen  or  silk  thread 

68 


GROWTH.  09 

is  tied  to  the  tip  of  a  shoot  of  a  Coleus,  Tomato,  or  Potato,  or 
leaf  of  a  Narcissus  grown  in  a  pot.  The  plant  is  set  directly 
under  the  pulley  and  the  string  is  passed  over  the  pulley,  and 
attached  to  this  end  is  a  weight  G  of  one  gram  or  more  to  keep  the 
threat  taut.  As  the  plant  grows  in  length  it  allows  the  weight 
G  to  descend,  turning  the  pulley  as  it  does  so.  The  pointer  is  at- 
tached directly  to  the  pulley,  and  as  the  elongation  takes  place  it 

FIG.  64. 


Lever  auxanometer.     (After  Oels.)     Z,  lever  ;  g,  balance-weight  on  lever  ; 
G,  counterpoise  to  keep  the  string  taut  ;  /,  string. 

passes  downward  along  the  scale.  Since  the  length  of  the  pointer 
from  the  center  of  the  pulley  is  16  cm.  and  the  radius  of  the  pulley 
is  2  cm.,  the  amount  of  growth  is  magnified  8  times.  The  apparatus 
is  set  up  with  the  pointer  at  zero  on  the  upper  end  of  the  scale. 
Observations  of  its  position  should  be  made  at  least  three  times 
daily.  A  growth  extension  of  i  to  5  cm.  daily  may  be  expected 
under  favorable  circumstances. 


EXPERIMENTAL   PLANT  PHYSIOLOGY. 


FIG.  65. 


EXPERIMENT  87. 

MEASUREMENT  OF  ROOT  EXTENSION. 

Germinate  Pea,  Bean,  or  Squash  seeds  until  the  primary  roots 
are  2  cm.  in  length.  Place  one  of  the  seed- 
lings  in  the  bowl  of  a  thistle-tube  or  small 
&  funnel,  with  the  root  depending  downward  in 
the  tube.  Cover  the  seedling  with  moist 
cotton  and  place  the  bottom  of  the  tube,  in 
a  vessel  of  water.  By  means  of  India  ink 
mark  off  intervals  of  2  mm.  on  the  tube,  and 
set  the  whole  apparatus  in  a  light  equal  in 
all  directions.  Note  the  position  of  the 
root-tip  at  least  twice  daily.  A  growth  of  4 
to  20  mm.  in  a  day  may  be  expected. 

Focus  a  horizontal  microscope  with  a 
power  of  25  diameters  on  the  extreme  tip  of 
the  root.  It  will  be  seen  to  move  slowly 
across  the  field  of  view.  (Fig.  65.) 


EXPERIMENT  88. 

MEASUREMENT    OF   GROWTH    INCREASE   BY    WEIGHT. 

Select  a  young  Squash  or  Pumpkin  which 
has  attained  a  diameter  of  a  few  centi- 
meters and  place  on  the  pan  of  a  druggists' 
balance  (Fig.  24),  with  the  vine  supported  in 
such  a  manner  that  it  bears  as  little  weight  as 
possible  on  the  balance.  Place  in  the  second 
pan  sufficient  weights  to  establish  an  equilib- 
rium. Equalize  the  scale  morning,  noon, 
and  evening,  and  the  amount  of  increase  may 
be  directly  obtained.  During  the  period  of 
most  rapid  growth  the  daily  increase  will 
amount  to  200  to  700  grams.  At  times  the 

Seedling  of  Squash  in  a  Weight  of    the  fruits   will    be   found    less    at 
thistle-tube.  (Detmer.)  ...  . 

noon  than  in  the  morning  owing  to  excessive 

evaporation  of  water  from  its  surface  and  that  of  the  leaves. 

53.  Grand  Period  of  Growth. — With  regard  to  growth  three 
regions  may  be  observed  in  any  organ  composed  of  many 
cells :  one  in  which  new  cells  are  constantly  forming,  as  in 
the  tips  of  roots  and  shoots  ;  another  in  which  the  cells  are  in- 


GRO  WTH. 


FIG.  66. 


creasing  in  size,  and  a  third  in  which  the  cells  have  attained 
their  full  size  and  maturity.  The  time  inclusive  of  the  forma- 
tion and  enlargement  of  a  cell  is  termed  its  grand  period  of 
growth.  In  the  case  of  an  organ,  this  period  includes  the  times 
from  the  formation  of  all  of  its  cells  to  their  maturity.  All  of 
the  cells  are  not  formed  at  the  same  time  and  do  not  reach 
maturity  at  the  same  time.  The  portion  containing  the  cells 
which  are  enlarging  most  actively  is  designated  the  zone  of 
maximum  growth.  This  zone  is  constantly  changing  its  posi- 
tion, as  may  be  seen  in  the  following  experiments. 
\S  EXPERIMENT  89. 

ZONE   OF   MAXIMUM    GROWTH    OF    ROOTS. 

Select  a  healthy  seedling  of  Pea,  Bean,  or  Squash  with  a  root- 
let 2  cm.  in  length.  With  a  pointed 
camel's-hair  brush  mark  off  ten  in- 
tervals i  mm.  apart.  Place  the  seed- 
ling in  a  thistle-tube  as  in  Experiment 
87.  Set  where  it  may  receive  an  equal- 
sided  illumination.  In  twenty-four 
hours  observe  the  length  of  the  inter- 
vals. The  fourth,  fifth,  sixth,  and 
seventh  from  the  tip  will  be  found  to 
have  elongated  much  more  than  any 
of  the  others.  Twenty-four  hours 
later  the  terminal  division  will  have 
partaken  of  this  elongation,  showing 
that  the  zone  of  maximum  growth 
moves  steadily  toward  the  tip.  Now 
follow  the  growth  of  the  second  inter- 
val. At  the  beginning  of  the  experi- 
ment it  elongates  somewhat  slowly  at 
first,  then  more  rapidly,  until  it  is  grow- 
ing more  rapidly  than  any  other  por- 
tion of  the  root.  Its  rate  then  de- 
creases until  it  finally  ceases.  In  the 
mean  time  the  next  interval  toward  the 
tip  begins  to  increase  in  rapidity, 


Seedlings  of  Pea.  (Sachs.) 
Showing  zone  of  maximum 
growth. 


72  EXPERIMENTAL   PLANT  PHYSIOLOGY. 

and  about  the  time  the  previous  one  has  begun  to  lessen  its  rapidity 
of  growth,  it  has  reached  its  maximum.  In  this  manner  the  zone  of 
maximum  growth  progresses.  (Fig.  66.) 


EXPERIMENT  90. 

ZONE    OF   MAXIMUM    GROWTH    OF    STEMS. 

Growth  of  stems  may  be  observed  in  the  same  manner  as  in  the 
last  experiment.  The  elongation  of  the  natural  divisions,  the  inter- 
nodeSi  can  be  measured  and  compared  with  one  another.  The 
elongating  part  is  greater  than  in  roots — 35  millimeters  in  the  Bean. 
The  internodes  vary  in  length  ;  the  middle  ones  are  the  longest. 
Satisfactory  results  may  be  attained  by  the  measurement  of  centi- 
meter intervals  on  the  stem  of  Bean  or  Corn.  Compare  movement 
of  zone  of  maximum  growth  with  that  in  roots. 

EXPERIMENT  91. 

ZONE   OF   MAXIMUM    GROWTH    OF   LEAVES. 

Cultivate  Gourd  or  Tobacco  plants  in  large  pots,  and  after  some 
leaves  have  been  formed  place  them  under  large  bell-jars,  and  set 
in  light,  but  not  in  direct  sun  light,  in  a  temperature  as  nearly 
constant  as  possible.  Before  doing  this  mark  off  on  the  petiole  or 
midrib  of  a  young  leaf  a  scale  as  above.  Compare  observations 
with  results  of  above  experiments. 

54.  Influence  of  Light  on  Growth. — While  light  is  necessary 
for  the  formation  of  food  by  photosynthesis,  and  for  the  per- 
formance of  certain  other  functions,  it  at  the  same  time 
generally  retards  growth.  Only  the  blue-violet  end  of  the 
spectrum  exercises  this  retarding  influence.  By  reason  of 
this  influence  the  maximum  growth  of  a  great  number  of  plants 
occurs  after  they  have  been  deprived  of  light  for  the  longest 
period,  which  is  in  the  morning,  or  just  before  daylight. 
Temperature  is  generally  more  favorable  to  growth  during  the 
afternoon,  and  as  a  consequence  the  plant  grows  rapidly  at 
this  time  also.  In  fact  the  maximum  growth  often  occurs  then. 


GROWTH.  73 

Light  also  influences  the  form  and  size  of  the  cells,  as  well  as 
of  the  entire  plant.     (See  §40.) 

EXPERIMENT  92. 

GROWTH    OF    SEEDLINGS    IN    DARKNESS. 

Grow  seedlings  of  Cucurbita  (Squash)  in  similar  pots,  some  of 
which  are  set  in  the  light,  and  others  JTIG 

are  covered  by  a  pasteboard  box,   at 
the  same  temperature.     The  latter  do 
not  develop  normally  ;  the  shoot  axes 
are  much  extended,  and  form  only  im- 
perfect leaves,  which  assume    an   up- 
right   position.     (Fig.    67.)     All    parts 
of   the  plant  are  pale  and   dispropor- 
tionately tender.     The  lignification  of 
the  wood  is  hindered,  in    consequence  Cucurbita  seeaimgs.  (Detmer.) 
of  which  there  is  no  opposition  to  the      a,    grown    in    darkness  ;    b> 
extension  of  the  tissues  by  the  turgor     erown  in  Hght- 
stretching  of  the  parenchyma-cells. 

Remark. — Care  must  be  taken  in  this  experiment  that  both  plants  do 
not  stand  in  the  sunlight,  otherwise  an  abnormally  high  temperature  will 
arise  in  the  pasteboard  box,  and  thus  the  relations  of  temperature  will  be 
altered. 


EXPERIMENT  93. 

COMPARISON    OF    GROWTH    OF    SEEDLINGS    IN    LIGHT   AND    DARKNESS. 

Germinate  a  number  of  Peas  in  a  pan  of  moist  sawdust  until  the 
main  roots  are  2  cm.  long.  After  the  roots  of  several,  as  nearly 
alike  as  possible,  have  been  marked  with  a  scale,  as  in  Experiment 
89,  place  some  in  the  light,  and  others  under  a  pasteboard  box,  over 
spring-water.  It  will  be  found  that  the  growth  in  light  is  less  than 
in  darkness.  At  the  same  time  the  daily  period  of  growth  can  be 
observed. 

55.  Influence  of  Light  upon  the  Anatomy  of  the  Leaf. — The 
leaves  of  common  trees  have  in  the  upper  side  a  closely- 
arranged  layer  of  cells  rich  in  chlorophyll  (palisade  par  enchyma\ 
and  in  the  lower  side  a  loosely-arranged  tissue  poor  in  chloro- 
phyll (spongy  parenchyma).  This  arrangement  depends  upon 
the  influence  of  light.  Shaded  leaves  exhibit  another  struc- 


74  EXPERIMENTAL   PLANT  PHYSIOLOGY. 

ture,  and  leaves  which  have  been  artificially  twisted,  so  that 
the  lower  side  is  exposed  to  the  light,  reverse  the  arrangement 
of  these  two  kinds  of  parenchyma. 

EXPERIMENT  94. 

INFLUENCE  OF  LIGHT  ON  THE  STRUCTURE  OF  LEAVES. 

Turn  and  fasten  young  leaves  of  the  Beech  (Fagus  sylvatica)  so 
that  the  under  side  is  exposed  to  the  light.  When  mature  they 
show  palisade  tissue  in  the  side  now  above,  and  spongy  parenchyma 
in  the  side  turned  away  from  the  light,  as  may  be  seen  on  examina- 
tion with  the  microscope. 

EXPERIMENT  95. 

DEVELOPMENT    OF    FLOWERS    IN    DARKNESS. 

Enclose  a  young  inflorescence  of  Scarlet  Runner  or  Morning- 
glory  in  a  pasteboard  box  or  bag  of  thick  black  cloth.  The  flowers 
and  fruit  will  develop  normally  in  the  darkness  thus  secured. 

56.  Influence  of  Gravity  and  Light  on  the  Formation  of  Organs. 
— Light  and  gravity  influence  the  origin  and  demarkation  of 
the  forms  of  organs  in  a  very  remarkable  manner.  If  a  twig 
of  Willow  or  some  other  plant  is  placed  in  a  damp  chamber, 
root  and  leaf  buds  will  develop  under  the  bark.  If  the  twig  is 
placed  in  an  upright  position,  the  roots  will  develop  below  and 
the  leaves  above.  This  "  polarity  "  is,  according  to  Vochting, 
due  to  light  and  gravity.  The  action  of  light  induces  the 
formation  of  shoots  on  the  illuminated  side,  and  roots  on  the 
shaded  portion.  That  gravity  acts  in  a  similar  manner  may 
be  shown  under  other  conditions.  If  a  Willow  twig  is  rapidly 
turned,  like  the  diameter  of  a  wheel  (Experiment  68),  shoots 
will  be  formed  near  the  center  of  revolution,  and  roots  at 
the  peripheral  ends.  The  symmetry  of  flowers,  according  to 
Vochting's  researches,  is  due  to  the  influence  of  gravity. 


GROWTH. 


75 


EXPERIMENT  96. 

COMPARISON    OF    THE    GROWTH    OF    CUTTINGS    IN    LIGHT   AND    DARKNESS. 

Fasten  a  Willow  twig  in  an  upright  position  in  a  covered  glass 
cylinder  containing  some  water,  and  set  in  the  sunlight.     It  develops 
roots  below  and  shoots  above.     Another  twig  treated  in  the  same 
FlG   6S        manner  but  placed  in  the  dark   acts  similarly.     (Fig. 
68.) 

EXPERIMENT  97. 

INFLUENCE  OF  LIGHT  ON  THE  FORMATION  OF  ROOTS  AND  SHOOTS. 

Set  up  the  experiment  as  above,  but  place  the  cylin- 


FIG.  69. 

ii 


der  in  a  pasteboard  box  which 
admits  light  at  the  side  through 
a  long  slit.  Roots  develop  on  the 
side  of  the  shoot  away  from  the 
light,  and  shoots  on  the  illumi- 
nated side. 


EXPERIMENT  98. 

DEVELOPMENT    OF    ROOTS   AND   SHOOTS 
IN    A    REVERSED    POSITION. 

Suspend  a  Willow  twig  in  a 
reversed  position  in  a  glass  cylin- 
der furnished  with  water,  in  the 
light.  A  contest  arises  between 
the  specific  tendency  of  the  twig 
to  form  shoots  on  the  original 

68.  Willow  twig  in  normal  position.    U?Per  end>  and  rOOtS  °n  the  lower 
(Hansen.)    a,  shoots  ;  6,  roots.        end,  and  the  influence  of   light 

69.  Willow  twig  in  reversed  position.    and  gravity   which    directly  op- 
(Hansen.)    a,  original  upper  end  ;    pose    it.       At    first    the    habit    of 


by  original  lower  end. 


the  plant  prevails,  and  roots  are 


formed  on  the  upper,  shoots  on  the  lower  end.  Then  the  influence 
of  the  physical  forces  is  manifested  by  the  development  of  roots  on 
the  lower  and  shoots  on  the  upper  end  of  the  twig.  (Fig.  69.) 

57.  Influence  of  Temperature  on  Growth. — For  every  plant 
there  are  five  important  temperature  divisions  :  1st,  destructive 
cold,  a  low  temperature  producing  death  by  the  disorganiza- 
tion of  the  protoplasm ;  2d,  specific  zero,  which  arrests  the 
activity  of  the  protoplasm,  but  does  not  necessarily  result  in 


?  EXPERIMENTAL   PLANT  PHYSIOLOGY. 

damage  to  the  organism ;  3<d,  optimum  temperature,  in  which 
normal  development  proceeds  ;  4th,  maximum  temperature, 
at  which  the  protoplasmic  activity  comes  to  a  standstill  without 
necessarily  injuring  the  organism  ;  5th,  destructive  heat,  pro- 
ducing death  by  disintegration  of  the  protoplasm.  These  divi- 
sions vary  greatly  with  each  species. 

58.  Sources  of  Heat. — The  temperature  of  any  plant  is  the 
result  of  the  heat  it  receives  from  several  sources.     A  portion 
comes   directly  from    the    heat-rays  of  the   sunlight,  as   well 
as  from  the  light-rays  which  it  is  able  to  convert  into  heat  by 
means  of  chlorophyll,  anthocyanin,  and  other  coloring  matters. 
Another  portion  is  received  from  the  soil,  which  is  generally 
more   constant  in   temperature   than  the  air.     According   to 
Kerner  the  soil  of  a  mountain  at  a  height  of  2200  meters  is 
3.6°  C.  higher  than  the  surrounding  air.     Another  and  by  no 
means  unimportant  source  of  heat  is  the  combustion  of  the 
carbon  compounds  in  the  plant.     (See  Experiment  53.) 

59.  Influence  of  Temperature  on  Geographical  Distribution. — In 
consequence  of  the  obliquity  of  the  ecliptic,  no  place  on  the 
earth  has  the  same  temperature  during  the  entire  year,  disre- 
garding even  the  changes  of  day  and  night.     Fluctuations  in 
temperature  vary  greatly  with  the    locality :    it  is  greater  in 
valleys  and  at  the  poles  than  it  is  on  mountains  and  at  the 
equator.    Fluctuation  further  depends  upon  the  continental  or 
oceanic  position  of  a  place.     Again,  between  the  elevated  cold 
regions  of  the  warmer  zones  and  the  polar  regions  there  is  the 
difference  of  short  period  of  daylight  and  the  long  summer  on 
one  hand   and   the  longest   period   of   daylight,  and  a  short 
summer  on  the  other.     These  conditions  of  temperature,  to- 
gether with  those  of  rainfall  and  soil,  are  the  most  important 
factors  in  the  geographical  distribution  of  plants.    The  regions 
which  are  not  subject  to  extremes  of  temperature  will  be  found 


GROWTH.  77 

most  suitable  for  the  greater  number  of  species.  While  some 
species  of  plants  thrive  with  a  low  summer  heat  if  the  tempera- 
ture does  not  sink  to  the  destructive  point  in  winter,  others 
endure  a  low  temperature  in  winter  very  well  if  the  tempera- 
ture ascends  high  enough  in  summer  to  permit  normal  fruit- 
formation. 

60.  Freezing  of  Plants. — Formerly  it  was  believed  that  the 
cell-sap  was  frozen  by  cold,  that  by  the  resultant  expansion 
the  cell-walls  were  torn,  and  in  this  way  the  plant  was  killed. 
It  has,  however,  been  demonstrated  that  a  mechanical  destruc- 
tion of  the  cell  by  rupture  does  not  take  place,  for  the  ice- 
formation  goes  on  only  in  the  intercellular  spaces,  or,  in  the 
simpler  plants,  in  the  water  thrown  out  around  the  plant.  It 
is  therefore  now  held  that  death  by  cold  is  the  result  of  a 
chemical  process,  which  can  occur  at  a  temperature  even  above 
freezing-point. 

Rapid  or  slow  thawing  of  frozen  plants  has  no  influence 
upon  the  life-energy  of  the  plants.  If,  however,  frozen  plants 
which  are  not  killed  are  thawed  slowly,  the  cells  can  reabsorb 
the  water  from  the  melting  ice-crystals  around  them  and  regain 
their  former  turgor.  If  thawed  rapidly,  a  portion  of  the  water 
of  the  ice-crystals  is  evaporated  or  driven  away,  and  the  cells 
cannot  regain  their  turgor.  When  a  plant  remains  frozen  for 
some  time,  the  water  slowly  evaporates  from  the  crystals  and 
the  plant  is  eventually  dried.  Therefore  frozen  plants  may  be 
killed  by  loss  of  water,  either  through  continued  cold  or  rapid 
thawing. 

Salt  solutions  freeze  at  lower  temperatures  than  pure  water, 
and  in  their  freezing  the  water  is  separated  out  in  the  form 
of  crystals.  Cell-sap,  a  solution  of  several  stable  substances  in 
water,  acts  similarly,  and  plants  may  therefore  endure  a  tempera- 
ture many  degrees  below  freezing-point  without  being  frozen. 


78  EXPERIMENTAL   PLANT  PHYSIOLOGY. 

EXPERIMENT  98. 

FREEZING    OF   A    SALT    SOLUTION. 

Partially  freeze  a  solution  of  potassium  bichromate  or  copper 
sulphate.  The  frozen  portions  are  distinguished  from  the  concen- 
trated fluid  by  the  paler  color.  The  freezing  begins  at  a  tempera- 
ture a  few  degrees  below  zero  C. 

EXPERIMENT  99. 

•^FREEZING    OF   A    BEET. 

Place  a  section  of  a  Beet,  a  centimeter  thick,  well  washed  and 
dried,  in  a  dish  covered  with  a  glass  plate  to  prevent  evapora- 
tion, at  a  temperature  of  6  degrees  below  zero  C.  When  the 
section  is  frozen,  the  surface  will  be  covered  with  a  layer  of  ice,, 
which  when  examined  with  the  microscope,  at  a  temperature  below 
zero,  will  be  found  to  consist  of  parallel  crystals.  A  very  heavy  ice- 
layer  is  found  on  the  under  side  of  the  section,  where  it  has  been 
in  contact  with  the  dish.  The  ice  is  not  colored  red,  proving  that 
not  cell-sap  but  pure  water  drawn  from  the  cell  has  been  frozen. 
No  rupture  of  the  cell-wall  occurs  in  the  freezing  of  living  cells,  as 
would  be  the  case  if  the  enclosed  fluid  were  frozen. 

EXPERIMENT  100. 

FREEZING    OF    SPIROGYRA. 

Freeze  some  Spirogyra  filaments  in  a  drop  of  water  on  a  glass 
slide.  After  thawing  no  rupture  of  the  cell-walls  appears. 

EXPERIMENT  101. 

FREEZING    OF    POTATOES. 

Place  some  Potatoes  in  a  temperature  of  5  to  10  degrees  below 
zero  centigrade,  over  night.  They  are  frozen  hard,  and  upon  thawing 
become  very  soft,  allowing  the  sap  to  be  forced  out  by  the  lightest 
pressure.  Their  power  of  germination  is  lost,  and  they  easily  rot. 
Whether  the  Potatoes  are  thawed  quickly  or  slowly  is  a  matter  of 
indifference. 

61.  Relation  of  Moisture  to  Freezing. — Low  and  high  tern- 
peratures  are  destructive  to  plants  and  plant-organs  in  propor- 
tion to  their  richness  in  water. 


GROWTH.  79 

EXPERIMENT  102. 

FREEZING    OF    PEA,    BEAN,    AND    WHEAT. 

Place  some  air-dry  seeds  of  the  Pea,  Bean,  or  Wheat  for  several 
hours  in  a  temperature  of  5  to  10  degrees  below  zero  centigrade. 
They  do  not  lose  the  power  of  germination,  as  may  be  shown.  The 
same  kinds  of  seeds  when  saturated  with  water  are  killed  by  this 
temperature,  and  are  unable  to  germinate. 

Remark. — Trees  behave  similarly.  In  winter,  when  they  contain  but 
little  water,  they  endure  a  high  degree  of  cold  ;  a  late  spring  frost  kills 
them,  because  the  trunks  and  twigs  are  full  of  sap. 

EXPERIMENT  103. 

EFFECT  OF  HIGH  TEMPERATURE  ON  SATURATED  SEEDS. 

Place  30  swollen  seeds  of  Peas  or  Wheat  for  a  quarter  of  an 
hour  in  water  at  a  temperature  of  60°  to  70°  C.,  and  then  place  in 
a  germinator.  They  do  not  germinate,  while  30  other  seeds  placed 
in  the  germinator  after  soaking  develop  normally. 

62.  Protoplasm  which  has  been  killed  by  low  or  high  tem- 
perature undergoes  molecular  changes  ;  it  then  becomes  per- 
meable to  acids  and  coloring  matters.  (See  §  60.) 

EXPERIMENT  104. 

ESCAPE    OF   CELL-SAP    OF   BEET    KILLED    BY    LOW   TEMPERATURE. 

Frozen  and  unfrozen  pieces  of  Beet  are  placed  in  water  ;  the 
first  colors  the  water  red,  the  latter  does  not.  The  protoplasm  of 
the  frozen  cells  allows  the  colored  sap  to  pass  through  it. 

EXPERIMENT  105. 

ESCAPE  OF  SAP  FROM  A  BEET  KILLED  BY  HIGH  TEMPERATURE. 

Perform  the  above  experiment,  using  pieces  of  Beet,  one  of  which 
has  been  in  water  at  a  temperature  of  60°  to  70°  C.  The  result  is 
the  same  as  in  Experiment  104. 

EXPERIMENT  106. 

ESCAPE    OF   CELL-SAP    CONTAINING    OXALIC    ACID    FROM    A    STEM    OF    BEGONIA. 
KILLED    BY    HIGH    TEMPERATURE. 

Place  two  pieces  of  a  petiole  of  Begonia  in  distilled  water  after 
one  of  them  has  been  treated  with  water  at  a  temperature  of  60°  to 
70°  C.  until  colorless.  Add  a  solution  of  calcium  chloride  to  the 
dishes  containing  the  pieces.  The  water  in  one  dish  remains  clear, 


80  EXPERIMENTAL   PLANT  PHYSIOLOGY. 

while  that  in  the  other  becomes  turbid  from  the  formation  of  oxa- 
late  of  calcium.  The  heated  portion  permits  the  escape  of  oxalic 
acid  which  it  contains. 

63.  Loss  of  Heat. — On  account  of  the  importance  of  warmth 
for  the  chemical  processes  in  the  building  up  of  the  plant, 
many  plants  possess  peculiar  adaptations  for  preventing  undue 
loss  of  heat. 

EXPERIMENT  107. 

ADAPTATIONS   TO    PREVENT    LOSS    OF    HEAT. 

Grow  seedlings  of  Helianthus  (Sunflower)  and  Cucurbita 
{Squash).  As  soon  as  the  cotyledons  are  raised  above  the  earth, 
it  may  be  observed  that  they  are  extended  during  the  daytime,  and 
during  the  coolness  of  the  evening  close  together  above,  whereby 
the  loss  of  heat  by  radiation  is  decreased.  (See  Experiment  75.) 

64.  Resting  Period. — It  is  known  that  the  winter  buds  of 
trees  and  shrubs  can  be  made  to  open  very  early  in  the  spring 
if  they  are  placed  in  a  warm  room  or  greenhouse.     In  this 
way,  shoots  cut  from  Syringa  vulgaris  (Lilac),  or  the  Willow, 
in  February,  may  be  given  an  early  development.     It  might 
be  inferred  that  these  plants  are  compelled  to  rest  by  the  winter 
•cold  and  need  only  heat  to  set  in  motion  their  normal  develop- 
ment.    This  is  not,  however,  entirely  true.     Experiments  have 
shown  that  the  winter  resting  period  is  necessary  for  the  plant, 
or  rather  that  it  has  become  accustomed  to  it  by  thousands 
of  years  of  habit.     It  is  on  account  of  this  acquired  habit  that 
buds  brought  into  a  warm  room  in  January  do  not  begin  to 
develop  before  March,  and  Potato-tubers  brought  into  a  warm 
room  in  the  autumn  do  not   begin  to   germinate    until  after 
a  resting  period  of   greater  or  less  duration.     Potato-tubers 
which  are  placed  in  a  temperature  of  zero  centigrade,  for  four 
weeks  immediately  after  digging  in  August,  upon  being  planted 
in   garden  soil  and  watered,  will   begin  the   development    of 
buds. 


GROWTH.  8 1 

EXPERIMENT  108. 

ACCELERATED    DEVELOPMENT    OF   SHOOTS. 

Cut  off  twigs  of  Syringa  (Lilac),  Cornus  (Dogwood),  Salix 
(Willow),  etc.,  in  several  winter  months  ending  with  February,  and 
place  them  in  water  in  a  warm  room,  or,  better,  under  a  bell-jar  to 
keep  them  moist.  The  development  of  the  buds  proceeds  accord- 
ing to  the  laws  given  above. 

65.  Correlation  Processes. — Not  all  the  shoots  of  a  plant  come 
to  full  development.     Only  the  strongest  and  most  useful  to 
the  whole  plant  develop,  while  the  others  either  perish  or  carry 
on  a  kind  of   "  sleep-life."     These   last   are   generally  styled 
latent  buds.     If  the  plant  is  robbed  of  a  "  concurrent  "  organ 
by  any  accident,  the  nourishment  heretofore  used  by  that  organ 
is  sent  to  a  latent  bud,  which  then  emerges  from  its  period  of 
rest  and  develops.     Such  phenomena  are  termed  correlation 
processes.     In  gardening  much  use  is  made  of  this  capacity  of 
the  plant ;  as,  for  example,  in  the  formation  of  thick  hedges, 
in  the  development  of  branches  and  flowers  FIG.  70. 

on  the  Fuchsia,  etc. 

EXPERIMENT  109. 

DEVELOPMENT   OF    LATERAL    SHOOTS    OF   THE    BEAN. 

Germinate  two  plants  of  Bean  in  pots.  Cut 
the  epicotyl  from  one  as  soon  as  it  appears 
above  the  ground.  Then  the  buds  in  the  axils 
of  the  cotyledons  develop  instead.  The  otherj 
plant  serves  as  a  means  of  comparison. 

EXPERIMENT  no. 

DEVELOPMENT  OF  LATERAL  BUDS  OF  THE  POTATO. 

Place  a   Potato-tuber    with    the   stem-scar 
underneath  in  a  warm  room  without  the  addi- 
tion of  water.     The  buds  near  the  top  develop. 
Cut   these  off   and  the   lower  ones   start   into    Sprouting  Potato, 
active  growth.     (Fig.  70.)  (Detmer.) 

66.  External  Mechanical  Force  Exerted  by  Growing  Organs. — 
The  growing  cells  of  plants  are  able  to  exert  a  pressure  on 


82 


EXPERIMENTAL   PLANT  PHYSIOLOGY. 


bodies  surrounding  them  which  may  amount  to  from  12  to  15 
atmospheres.  By  this  force  roots  and  other  fixing  and  ab- 
sorbent organs  are  driven  through  the  soil,  and  aerial  organs 
push  their  way  upward  through  the  air.  The  total  amount  of 
energy  used  in  the  performance  of  external  work  during  the 
lifetime  of  the  plant  is  very  great.  The  spore-bearing  cap  of  a 
Mushroom  has  been  known  to  lift  a  weight  of  160  kilograms. 
A  root  of  Larch  30  cm.  in  diameter  has  lifted  a  stone  1600 
kilograms  in  weight,  while  a  root  of  a  germinating  Bean  has 
exerted  a  lateral  pressure  on  the  soil  amounting  to  1.5-4  kilo- 
grams. All  growing  organs  expand  with  similar  force,  but  in 
the  examples  given  the  form  of  the  organ  is  such  as  to  utilize 
the  force  in  penetrating  the  substratum.  The  growing  fruit 
of  a  Cucurbita  is  capable  of  exerting  a  pressure  of  several 
FIG.  71.  thousand  kilograms,  though  it  ordi- 

narily meets  with  no  resistance. 
EXPERIMENT  in. 

POWER    OF    PENETRATION    OF    RHIZOIDS    OF    A 
HEPATIC. 

If  a  Hepatic  is  placed  on  several  folds 
of  moist  filter-paper  in  a  chamber  satu- 
rated with  moisture,  within  forty-eight 
hours  the  rhizoids  will  have  pierced  the 
filter-paper.  The  holes  through  which  the 
rhizoids  have  penetrated  were  certainly 
not  there  before.  The  fibrous  structure 
of  the  paper  is  so  dense  that  a  starch- 
grain  of  corn,  which  is  only  two  micro- 
millimeters  in  diameter,  cannot  find  its  way 
through,  yet  the  rhizoids,  which  are  10  to 

«    micromillimeters    in    diameter,    easily 
Force   exerted  by  growing  •" 

roots.     (Mangin.)  accomplish  it. 

EXPERIMENT  112. 

FORCE  EXERTED  BY  GROWING  ROOTS. 

To  a  small  upright  stand  attach  a  horizontal  arm  bearing  a  small 
wooden  pulley.     Fasten  a   scale-pan  to   a   cord  passing   over  the 


GROWTH.  $ 

pulley  to  the  other  end  of  which  is  attached  a  second  pan  contain- 
ing a  5-gram  weight.  Fill  the  first  pan  firmly  with  moist  sand,  and 
fasten  a  seedling  of  Bean  in  such  position  that  it  touches  the  sand. 
It  will  push  downward  into  the  sand  and  elevate  the  weight-pan. 
The  scale-pan  touched  by  the  root  may  be  suspended  directly  from 
the  horizontal  arm  by  a  delicate  spiral  spring,  omitting  the  pulley 
and  second  scale-pan.  As  the  root  grows  it  will  push  the  pan 
downward  as  before,  and  the  distance  through  which  the  scale- 
pan  moves  will  indicate  the  force  directly.  The  strength  of  the 
spring  can  be  determined  by  placing  weights  on  the  scale-panv 
(Fig.  71-) 


APPENDIX. 

ENGLISH   AND   METRIC   WEIGHTS   AND   MEASURES. 


LENGTH. 

micro-millimeter  =  y^-  millimeter  or  -fjfav  incn- 

millimeter  (mm.)  =  -fa  inch. 

centimeter  (cm.)  =  10  mm.  =  f  inch. 

decimeter  (dm.)  =  100  mm.  =  4  inches. 

rneter  =  1000  mm.  =  39^  inches. 

inch  =  25  mm. 

foot  =  305  mm.  or  30^  cm. 

yard  =  .91  meter. 

WEIGHT. 

i  gram  =  15-2-  grains. 

i  kilogram  =  1000  grams  =  32  oz.  Troy  or  35  J  oz.  Avoirdupois. 

i  oz.  Troy  =  31  grams. 

i  oz.  Avoirdupois  =  28  grams. 

i  Ib.  Avoirdupois  =  450  grams. 

CAPACITY    AND    WEIGHT. 

i  gram  =  i  cubic  centimeter  (cc.)  =  15^  grains. 
i  liter  =  1000  grams,  or  1000  cc.,  or  i  kilogram  =  35  J  oz.  Avoirdu- 
pois or  32  oz.  Troy, 
i  pint  =  20  oz.  Avoirdupois  —  567-^  grams  or  567-^  cc. 

CAPACITY  (VOLUME). 

i  liter  =  1000  cc.  =  i  cubic  dm.  —  if  pints. 

i  pint  =  36  cubic  inches  =  567^  cc. 

i  gallon  =  8  pints  =  4^  liters. 

i  cubic  foot  =  6  gallons  =  28^  liters. 

84 


APPENDIX.  85 

CENTIGRADE   AND    FAHRENHEIT    THERMOMETER    SCALES. 


30°  Cent.  = 

—  22°  Fahr. 

40°  Cent.  =  104°  Fahr. 

25°  Cent.  = 

-  13°  Fahr. 

45°  Cent.  =  113°  Fahr. 

20°  Cent.  = 

4°  Fahr. 

50°  Cent.  =  122°  Fahr. 

15°  Cent.  = 

5°  Fahr. 

55°  Cent.  =  131°  Fahr. 

10°  Cent.  = 

14°  Fahr. 

60°  Cent.  =  140°  Fahr. 

5°  Cent.  = 

23°  Fahr. 

65°  Cent.  =  149°  Fahr. 

o°  Cent.  = 

32°  Fahr. 

70°  Cent.  =  158°  Fahr. 

5°  Cent.  = 

41°  Fahr. 

75°  Cent.  =  167°  Fahr. 

10°  Cent.  = 

50°  Fahr. 

80°  Cent.  =  176°  Fahr. 

15°  Cent.  = 

59°  Fahr. 

85°  Cent.  =  185°  Fahr. 

20°  Cent.  = 

68°  Fahr. 

90°  Cent.  =  194°  Fahr. 

25°  Cent.  = 

77°  Fahr. 

95°  Cent.  =  203°  Fahr. 

30°  Cent.  = 

86°  Fahr. 

100°  Cent.  =  212°  Fahr. 

35°  Cent.  = 

05°  Fahr. 

110°  Cent.  =  230°  Fahr. 

INDEX   TO    PLANT   NAMES. 


Alga 6 

Anthemis 45 

Apple 26 

Aroids 45 

Asclepias 32 

Bvcteria 3,  n,  12,  44 

Balsamina 67 

Barley 48 

Bean,  5^  10,  27,  41.  52,  53,  59,  60, 

62,  66,  70,  72,  81,  83 

Beech 14,  74 

Beechdrops n 

Beet 78,  79 

Begonia 25,49,  79 

Bellis 45 

Birch 31 

Bryony 64,  66 

Buckwheat 5 

Cabbage 14 

Carduus 64 

Carrot 14 

Centaurea 64 

Cichorium 64 

Coleus 14,  32,  69 

Composite 45 

Corallorhiza n 

Coral-root 1 1 

Corn,  5,  8,   10,  20,   27,  34,  37,  52, 

60,  62,  72 

Cornus 81 

Cucurbita 73,  80,  82 

Cuscuta 10,  n 

Dahlia 20 

Dionaea 63,  65 


Dodder 10 

Dogwood 81 

Drosera 63,  65 

Elder 18,  33 

Elodea 36,  37,  38,  40 

Epiphegus n 

Erodium 67 

Euphorbia 32 

Fagus 74 

Fritillaria 52 

Fuchsia 81 

Funaria 42 

Fungus ii 

Geranium 14,  20,  42 

Gourd 66,  72 

Grape . . .  18,  19,  20,  36 

Grass 53,  66 

Helianthus 32,  59,  60,  So 

Hemp 44 

Hepatic 82 

Hyacinth 52 

Impatiens n,  14,  34,  67 

Indian-pipe 11 

Iris 14,  24 

Larch 82 

j    Leontodon 45 

Lilac So,  Si 

Liverwort 14 

Lonicera 29,  31,  32 

Mddia 45 

Malva 59 

Maple 59 

Marchantia 25 

Milkweed 32 

87 


88 


INDEX    TO   PLANT  NAMES. 


PAGE 

Mimosa 50,  62,  63 

Mistletoe 10,  n 

Monotropa n 

Morning-glory 74 

Mosses 14 

Mould ii 

Mushroom u,  82 

Mustard 8,  59 

Myosotis 49 

Narcissus 52,  69 

Nettle 19 

Oak.. 


14 

Onion 52 

Oxalis 60 

Passion-flower*, 65 

Pea,  5,  8,  9,  10,  16,  17,  27,  45,  46,  52, 
53,  55,  62,  66,  69,  70,  79 

Peony 49 

Phaseolus 53,  59 

Poplar 18 

Potato '. 26,  69,  80,  81 

Pumpkin 70 

Raspberry 22 

Rhubarb 18 

Rose 49 

Rosebush ^.  ..21,23 

Rust ii 


PAGE 

Salix... 8t 

Sambucus 18,  31,  34 

Scarlet  runner 74 

Seaweed 39 

Sensitive  plant 50,  63 

Sinapis 59 

Smut ii 

Sonchus 32 

Spirogyra 42,  78 

Spurge 32 

Squash 9,  64,  70,  73,  80 

Sundew , 65 

Sunflower,  18,  19,  20,  29,  32,  60,  76,  80 

Symphytum 23 

Syringa 80,  Si 

Toadstool ii 

Tobacco 72 

Tomato 14,  17,  41,  69 

Touch-me-not 34 

Tropaeolum 41,  49 

Tulip 52 

Vaucheria 42 

Wheat 5,  27,  46,  75 

Wild  balsam-apple 66 

Wild  lettuce 32 

Willow 18,  34,  74,  75,  80,  Si 

Yeast 44 


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